That often doesn't make much sense, since it's hardly Renegade to kill people who are trying to kill you. But I won't really complain because it gives us awesome stuff like "How about goodbye" and that thing on Mordin's loyalty mission.
Basically every time a Renegade interrupt kills someone, it's someone you were going to kill in combat anyway. Or someone providing tech support for the guys you're fighting.
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Mistake you're making - overlooking the fact that we might not want to be saved.