wrong. I am not a lawyer, but isn't self defense considered an "affirmative defense," which means the burden of proving self defense is actually on the person using that defense.
Presumption of innocence absolutely still exists if you are trying to prove "who dunnit." But this siutation is different. "Who dunnit" is not in question, what's in question is "why". In such cases, presumptions of innocence , imo, should not exist.
From a societal viewpoint, homicide in my opinion, is just different from other crimes. Anyone claiming justifiable homicide (including law enforcement officers) has to prove it is indeed justifiable. In my opinion, we can't and shouldn't have it any other way. You don't just kill anyone and expect the rest of us to automatically presume you have a good reason for it. That's madness! I think by and large, with some glaring exceptions of certain people, that's how laws are actually structured (but I am not a lawyer).
Think about it, do you really want to live in a society where anyone and everyone who kills another person should be presumed to always be justified? The armed robber or hired assassin who kills can claim it's justfiable, the burden of proof is on the state to prove it wasn't. That's just madness.