In some states, someone who uses deadly force according to an honest—but unreasonable—belief in the need to defend oneself from imminent and serious harm may have a “partial” defense. In these states, this partial defense may also be available where:
This partial defense is called imperfect self-defense. In those states that recognize it, it applies only to murder and attempted-murder cases. The defense is “partial” because it doesn’t result in an acquittal. Instead, it results in a conviction for a lesser charge.
For a murder conviction, the defendant must have killed with “malice.” Where available, imperfect self-defense negates that “malice” element. The effect is to reduce murder to manslaughter.
For much more on this doctrine, including additional circumstances where it sometimes applies, see Imperfect Self-Defense.