I don't think that there is a line we can draw, short of saying "no killing whatsoever," that can't be abused. If we say, "No killing unless they're judged guilty," then we leave it up to fallible lawmakers to make just laws, and fallible police and prosecutors to be honest and decent, and fallible jurors to try and turn this into a decision.

Or let's say we make the rule, "Only fascists can be murdered." But who decides if someone is a fascist? Is it someone who believes the ideology, or do they need power or authority to act on it as well?

Basically I don't think there is a system of rules that could be implemented that would not kill innocent people as a byproduct, unless the rule is just "no killing."