Dogs, cats, and other animals are treated like property under the law. That usually means that people who kill someone else's dog may have to compensate the owner, just as if they destroyed another kind of property that wasn't theirs. They could also face criminal charges, including animal cruelty or criminal property damage. But there are exceptions—certain circumstances when people have the legal right to kill a dog. Some of these exceptions are written into state and local laws, while courts have recognized others.
Most animal cruelty laws make it a crime to kill or injure animals "unnecessarily" or "without justification." The most obvious justification is self-defense or defending another person from harm. That doesn't necessarily mean, however, that you can shoot a dog just because it's growling or barking at you or it has bitten someone in the past. The general rule most courts follow: You must believe it's necessary to kill or injure the animal in order to prevent an immediate threat of serious injury—and that belief must be reasonable. (See, for example, Grizzle v. State, 707 P.2d 1210 (Okla. Crim. App. 1985).) Some states, like Georgia, have explicitly included this rule in their laws (Ga. Code Ann. § 16-12-40).
Are you allowed to kill a dog if it's not in the midst of attacking a person? That depends on the circumstances, as well as the language in any applicable state laws. For example:
Many states also have laws that make it legal for farmers or others to kill dogs that are chasing, harassing, or injuring their livestock or domestic animals—which may or may not include pets. (For more details, see "When Dogs Hurt or Chase Livestock.")
People generally aren't allowed to kill someone else's dog in retaliation for past attacks, unless there's an exception in the law. For instance, a California statute says that people have the right to kill any animals "known as dangerous to life, limb, or property" (Cal. Penal Code § 599c).
It's a different story when it comes to government authorities. Officials often have the right to kill dogs based on what they've done in the past, as long as they follow legal procedures—including giving the owners notice and the opportunity to challenge the government's proposed action. Local animal control officers usually have the authority to pick up, impound, and even destroy dogs that are a threat because of past behavior. And under the "dangerous-dog laws" in many states, authorities may—under certain circumstances—euthanize dogs that have been declared dangerous or vicious.
Courts have generally found that landowners don't have the right to kill dogs just because they're trespassing. Here again, there may be exceptions. For example, an Ohio statute says that it's not illegal for landowners to kill or injure animals while trying to keep them from trespassing or while driving them away from the property. However, the landowners must pay compensation to the animals' owners, minus the amount of any damage that the trespassing dogs caused. (Ohio Rev. Code Ann. § 959.04.)
Animal cruelty laws often specifically outlaw poisoning dogs on purpose, including putting out poison where you know a dog is likely to get into it. Most states don't make an exception for trespassing dogs, but many do exempt poisoning that's not malicious. (See, for example, N.Y. Agric. & Mkts. Law § 360; 510 Ill. Comp. Stat. Ann. § 70/6; Va. Ann. Code § 18.2-144.) Some states, such as California, specifically allow putting out poison on your own property to control predatory dogs or other animals, as long as you've placed conspicuous signs warning about the poison (Cal. Penal Code § 596).
If you've killed or injured someone else's dog—or another person has hurt your pet—you might want to consult with a lawyer. An attorney specialized in animal law or personal injury law should be able to explain how local law and recent court decisions apply to your situation, as well as your legal options. And if you're facing criminal charges for killing a dog, you'd be wise to contact a criminal defense attorney as soon as possible, in order to protect your rights.