Cops shoot man holding a knife: Justifiable or unjustifiable?

This is a fraction of what we are taught at the Academy, but it does apply to this discussion.

As you can plainly see, both use of deadly force in connection with an arrest and use of force for the prevention of crime apply to this particular case.
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Note the following in: “Use of deadly force in connection with an arrest”

“The U.S. Supreme Court stated the standard in 1985: "Where the officer has probable cause to believe that the suspect poses a threat of serious physical harm, either to the officer or to others, it is not constitutionally unreasonable to prevent escape by using deadly force. Thus, if the suspect threatens the officer with a weapon or there is probable cause to believe that he has committed a crime involving the infliction or threatened infliction of serious physical harm, deadly force may be used if necessary to prevent an escape, and if, where feasible, some warning has been given"”

Note the following in: “Use of force for the prevention of crime”

“Deadly force may be used to prevent a felony that threatens death or serious bodily harm, at least if the felony cannot otherwise be prevented (Restatement of Torts, 2d ed., secs. 141–143).”
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In the case of this story, the headline is skewed to make people think that the suspect was just holding a knife, when in reality, he was holding a women in a head lock and threatening to do her bodily harm with the knife (a deadly weapon by all standards).

The shoot was justified.

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Use of deadly force in connection with an arrest

As noted above, an officer may use as much force as is reasonably necessary, short of deadly force, to retain custody of a suspect. It follows that if the suspect resists, the officer may increase the force to counter the resistance. The officer has no duty to retreat as the force escalates, and if the force should ratchet up to the point where the suspect threatens the officer with death or serious bodily harm, the officer may use deadly force to retain custody (LaFave and Scott, sec. 5.10). "Deadly force" is defined as "force reasonably capable of causing death or great bodily harm" (Geller and Scott, p. 23); while it obviously includes the discharge of firearms, it may also include the use of chokeholds or even automobiles under some circumstances. Pointing a firearm without firing it or making any attempt to fire it is not in itself the use of deadly force.

The standard for the use of deadly force changes when the officer is pursuing a suspect but has not yet been able to arrest him. All the standards stated above are applicable to the use of deadly force to effect an arrest; the officer must reasonably believe that the use of such force is necessary. In addition, however, there is a further limitation under the Fourth Amendment upon the power of a law enforcement officer to use deadly force to effect an arrest. The U.S. Supreme Court stated the standard in 1985: "Where the officer has probable cause to believe that the suspect poses a threat of serious physical harm, either to the officer or to others, it is not constitutionally unreasonable to prevent escape by using deadly force. Thus, if the suspect threatens the officer with a weapon or there is probable cause to believe that he has committed a crime involving the infliction or threatened infliction of serious physical harm, deadly force may be used if necessary to prevent an escape, and if, where feasible, some warning has been given" (Tennessee v. Garner, 471 U.S. 1, 11–12 (1985)). Garner concerned the shooting of a suspect fleeing from a burglary who was not believed to pose any physical threat; in that case the Court held that the shooting was an unreasonable seizure of the person. The Garner standard is a substantial modification of the common law, which permitted an officer to shoot a fleeing suspect whom he had probable cause to believe had committed a felony, whether the felony was physically dangerous or not. The judgment that underlies the Garner standard is that while shooting a fleeing suspect may sometimes appear necessary to effect the arrest, the use of deadly force is disproportionate in cases where the suspect does not pose a physical danger to the officer or the community.

Even in a case where the use of deadly force is justified, the force used may be found to be excessive, for example, when it is not found to be necessary under the circumstances. In Burton v. Waller, 502 F.2d 1261 (5th Cir. 1974) cert den. U.S. 964, reh. den. 421 U.S. 39, the use of massive firepower in response to a suspected shot by a sniper in a civil disturbance was held to be the excessive use of force.
The standard for the use of deadly force to stop a suspect under international law, also squarely based in the basic principles of necessity and proportionality, is somewhat more restrictive than the standard under the Fourth Amendment. The United Nations Basic Principles for the Use of Force and Firearms by Law Enforcement Officials (UN Basic Principles ), which are widely adopted by police throughout the world, provide in Article 9 that: "Law enforcement officials shall not use firearms against persons except in self-defense or defense of others against the imminent threat of death or serious injury, to prevent the perpetration of a particularly serious crime involving grave threat to life, to arrest a person presenting such a danger and resisting their authority, or to prevent his or her escape, and only when less extreme means are insufficient to achieve these objectives. In any event, intentional lethal use of firearms may only be made when strictly unavoidable in order to protect life." In the Case of McCann and others v. UK, ECHR vol. 324 1995)), which concerned a response to a suspected terrorist attack, the European Court of Human Rights, applying the principle of necessity, held that it is not enough to justify every use of deadly force that the actors reasonably believe that the attackers present a threat to life; in addition, the official operation in response must be organized in such a way as to minimize the threat to life.

The standards for the protection of bystanders who may be injured by the lawful use of deadly force vary. At common law, if the action in connection with the arrest was justified, then an injury consequent upon that action would also be justified, and would not be a crime or even a tort (Restatement of Torts, 2d ed., sec. 75) in the absence of negligence on the part of the actor. Although the Model Penal Code proposed the more restrictive standard that the actor may not use deadly force unless he "believes that the force employed creates no substantial risk of injury to innocent persons" (Model Penal Code, sec. 3.07 (2)(b)(iii)), the standard has not been adopted. It is not clear how such a standard could be administered as a matter of the criminal law; if the force used were truly necessary and proportional, then it would seem that an element of criminality is missing from the act.

The standards for the use of deadly force in an arrest by a private person are generally more restrictive than the standards for law enforcement officers. A private person uses deadly force at his peril; he is not privileged to rely upon "probable cause." By the general rule under contemporary law, he may use deadly force only to arrest for a felony dangerous to life when the person arrested has committed the felony (Dressler, sec. 21.03 B2b; Restatement of Torts, 2d ed., sec. 143). On the other hand, since the Fourth Amendment does not restrict actions by private persons, it seems permissible for the states to retain the common law rule that permitted a person to use deadly force to arrest for any felony; the Michigan courts have done so (People v. Couch, 436 Mich. 414 (1990)). Nevertheless, this seems to be an undesirable standard, giving justification for disproportionate force when the crime is not a dangerous felony and unnecessarily encouraging vigilantism.

Use of force for the prevention of crime

At common law, reasonable force short of deadly force may be used by law enforcement officers or private persons to prevent a felony or a misdemeanor that involves a breach of the peace. Deadly force may be used to prevent a felony that threatens death or serious bodily harm, at least if the felony cannot otherwise be prevented (Restatement of Torts, 2d ed., secs. 141–143). The standards for the use of force to prevent crime overlap with those concerning self-defense and the defense of another, as well as the standards concerning the use of force for arrest. Thus, if the actor is the victim of the crime, or is aiding a victim, then standards concerning self-defense will support his actions; similarly the prevention of the crime will often entail an arrest of the offender.

By the standards established in the Garner case, discussed above, it appears that the standards for the use of deadly force by law enforcement officers, limiting their discretion to use deadly force in the prevention of crime to cases of life-threatening felonies, are required by the Fourth Amendment; to use deadly force to prevent a felony that does not threaten life would be disproportionate to the crime and an unreasonable seizure of the person. The powers of private persons to use deadly force, however, not being controlled by the Fourth Amendment, may be more expansive than the powers of law enforcement officers. The common law permits an actor in his home, after giving a warning, to repel an intruder with deadly force, and some states, including Louisiana and New York, retain versions of this rule; New York, for example, permits the use of deadly force to terminate a burglary (N.Y. Penal Law, sec. 35.20(2)). The Model Penal Code permits the use of deadly force to prevent dispossession from the dwelling when the attempted dispossession is not under a claim of right (Model Penal Code, sec. 3.06 (3)(d)(i)). Some states have taken the contrary position that rules similar to the common law rule are too permissive, because they would authorize the use of deadly force under circumstances where it may be disproportionate to the crime, and have permitted the use of deadly force only when the intrusion is reasonably believed to threaten life (LaFave and Scott, sec. 5.9).