We're letting our will to survive force us to run from accountability and rationality, as usual.
You own the effects of the actions you control. That is the libertarian position. Culpability is not a pie of fixed size that must be divided among participants or assigned only to the person at the top of some baddie hierarchy. It accrues to those with agency over their own actions. Duress may spread the pie around more, but it does not remote-control your brain. One still chooses and still acts, and therefore remains responsible for the means chosen.
B’s right not to be killed constrains C the same way it constrains anyone else. A’s aggression against B, without B’s consent, does not cancel B’s self-ownership nor enslave B’s body to C’s disposal. If C’s only means of stopping A foreseeably kills B, then C’s act is still an act against B.
Rapists would be overjoyed to hear that ones failure to sufficiently resist erases one's self-ownership and consent.