What is an idiom for this whimsical turn of events?

Question

What would be the most appropriate expression (idiom or phrase) to describe a situation where one person, usually innocent but related to the other person in some way, becomes the unfortunate target of some sinister plot aimed at the other person?

Movies abound in such tropes where A, usually the polar opposite of B, for no fault of his has to bear the brunt of B's underhanded dealings, the cost borne more often than not being a very high one— A loses his life; B avenges his death afterwards, and all that jazz.

Just to make it a bit more clear, targetting A is seldom the intent of the party who murder him. He gets murdered mostly because of a misconception or some such factor that suddenly supervenes as if to save A.

Please note that whipping guy, scapegoat, Aunt Sally, and other such words— these ain't what I'm looking for. I have also already ruled out collateral damage lest anyone should be tempted to post this one as answer, for obvious reasons.

I hope I've made myself sufficiently clear.

Answer 1

You got me sweating, exceptional question. I rummaged through some resources and gathered that frustratingly enough there is no to such an extent specific singular word. I've piled some synonyms that can point you to the right direction though:

Nouns: Obviously victim. Sufferer, casualty, fatality. Martyr may be a little inappropriate, due to him/her being uninvolved.

Adjectives: incidental

There is also a phrase I still hold on to: Unwitting victim of circumstance. Circumstance here can be replaced with plight, predicament, ploy, machinations or conspiracy even to convey ill intent.

Hope this helped you at least in the slightest.

Answer 2

In your scenario, you might say that A took the fall or B, or took the hit for B.

You might even call A the fall guy, except you suggested that B avenges A's death. Usually calling someone like A "the fall guy" suggests that B set A up to take the fall.