This line sounds peculiar, I would appreciate your opinions (it uses the verb 許される)

Question

First of all, to give you some context, there are 3 characters that will be referred to as A, B, and C. The following lines are from a dialogue between A and B about C. The character C has done things that are considered wrong, unforgivable actually, and that's why in the dialogue there is 許される.

Character A does not approve of what C did. However, B is grateful; he is on the side of C.

B: むしろ俺は感謝してるっスよ。
A: 許されることじゃねーだろ。
B: 許されるって誰に？つーか…なんでそんな無下にできんスか？

I'm fairly certain that A is saying something like "But, it's unforgivable."
"That sort thing shouldn't be allowed." (referring to C's actions)
The subject are his actions, right? I say this because if A had wanted to say "I can't forgive/allow/him for what he does", I believe (あいつを）許せない would have worked or 許されない.

Regarding B's line where he says 許されるって誰に？ if it sounds like he doesn't approve of what A said, and given that he also quotes what he said (with the use of って), personally I consider that it would have sounded better if B had also negated the verb, as in, to say: "Unforgivable, you say? By whom?"

Would have made sense in my opinion and much easier to translate, but it's not like that and I'm not sure what B is implying.

Answer 1

personally I consider that it would have sounded better if B had also negated the verb

If Person A had said 許されないことだろ, this is true. But って is a quoting particle, and what he actually said is 許される(ことじゃない), not 許されない.

Literally:

A: 許されることじゃねーだろ。
 It's not something that's to be forgiven.

B: 許されるって誰に？
 "Forgiven", by who?
 _{(implies "Regarding this matter, no one allows or disallows anything.")}

I think English speakers won't say "Unforgiven by who?" in this context, either.