Eloin

I will start out by quoting Kalessin: One must be in a position of power to show mercy. The question of self-defense is not a mercy question simply because one is not in a position to show mercy. The definition of mercy (oxford desk dictionary) is "compassion or forbearance shown to enemies or offenders in one's power."
This aspect of a position of power needed to show mercy is reflected in the phrase "at the mercy of"...as in to be wholly in the power of someone or something.

Mercy is appropriate in all situations because is it from a position of power to someone who is incapable of defending themselves. But showing mercy does not imply letting the perpetrator of evil go free to continue to do evil, nor is mercy the same thing as pardon. The state may show a murderer mercy by not imposing the death sentence, but still imprison the murderer (and not show pardon).

Look at the self-defense issue from a slightly different angle. The perpetrator/attacker is not attacking you, but is attacking a 3 year old child. Would it be mercy to let him harm the child rather than stopping him? Would you tell the child's mother that you had let her child be harmed/molested/whatever because you were being merciful? One has a right to self-defense.