In self-defense theory, there is a special situation. The aggressor attacks the defender by nonlethal means, and the defender defends him with lethal force; then, the aggressor fails to fulfill his duty to withdraw from the conflict and, instead, proceeds directly to self-defense with lethal force, which results in the death of the initial defender. The question of how to convict the aggressor for his homicidal act in such a situation becomes controversial. There are three conclusions to the problem in U.S. criminal law, and they are murder, innocence, and voluntary manslaughter. To prove the voluntary manslaughter conclusion, there are three paths: failure to satisfy the elements of murder, defense of provocation, and imperfect self-defense. Through theoretical and practical analysis, in this situation, the aggressor should be convicted of voluntary manslaughter, and the imperfect defense theory can provide a sound theoretical basis for this conclusion.