What is imperfect self-defense?

If someone comes running at you with a knife, the law doesn't expect you to do nothing and get stabbed to death. You're allowed to defend yourself and if, in defending your life, you kill your attacker, that is arguably okay. After all, you were defending yourself from a life or death situation. This type of situation would be considered a perfect example of self-defense, meaning if you were charged with the homicide, you could make a valid claim of self-defense.

Similarly, if you see someone else being attacked by someone with a knife and you help defend them resulting in the attacker being killed, your "defense of others" is just as legally justified as "self-defense."

But what happens when you mistakenly believe there is a threat to your life or the life of another person and, based on that mistaken belief, you kill someone? This is where we arrive in the land of "imperfect self-defense." It still means you're a bad person for killing an innocent person, but because you killed them based on a legitimate (though flawed) belief that they were going to cause you or someone else serious harm, you are not as bad of a person as someone who purposefully intends to kill innocent people.

For this reason, someone who commits an imperfect self-defense can have their homicide reduced to voluntary manslaughter instead of murder.

Note that there are two types of "unreasonable mistaken belief." The first type is when you have a an unreasonable perception of the threat (which we discussed above), the second is when you mistakenly use too much force. For example, if someone angrily punches you and, in response, you stab them — technically you defended yourself, but you arguably went a little nuts. Had you punched them back, or grabbed their arm to prevent them from punching you, your self-defense would have been justified. But by escalating a punch into a situation where you stab someone to death, your self-defense was imperfect and, as a result, you'll be liable for the homicide to some degree, even if not entirely murder.