The real question here is why he would even question his beliefs. Picture this situation:I am going to ask your forgiveness and say I ain't reading all that. So if I miss a point you made, sorry.
I realize my position as reader gives me certain information the protagonist doesn't. That doesn't really address the issue I have with the fact that the escalation doesn't make sense.
There's nothing to gain from him being killed after getting booted from the party. It doesn't harm their reputation he is alive, everyone thought he was dead weight at that point. He certainly had no way to threaten them physically or other wise. It's either a net 0 or a loss if they were caught.
Coupled with their long history, even if the relationship was soured it wouldn't be crazy to want some confirmation, even if it's just a verbal confrontation.
I DO think that the initial betrayal allows him to make that assumption, especially in the moment, but two years on he doesn't even question events? Especially she's apparently not trying to kill him anymore? She attempted murder because she didn't keep her promise to be big and famous together doesn't really hold up to scrutiny, it does makes sense when it's fueled by his own anger and possibly self loathing for not being "good enough".
MC already had low self-esteem from the getgo due to him feeling inferior to the party he was in.
He's then kicked out with only the "we want to move up ranks and you're an obstacle" spiel.
He's then ignored by the former party for 2 months, while getting verbally abused by virtually everyone in the guild(which is weird considering the guild master is apparently very close to him, but that's beside the point).
At this point instead of trying to talk anything out, the girl that kicked him out suggests to him a gathering spot for herbs.
Said point ends up with him almost dead and costs him an eye, then for the next 2 years, he gets no contact whatsoever with the direct responsible for his almost death.
How could you suppose someone on this situation would start questioning anything? All this looks like is someone guilty running away from facing you.