it's not like he can't but the revengers were like family to him so even if he wants to die so bad he wants to respect their dying wishes first.
It's the survivor's guilt. His conscience is preventing him from killing himself because of the guilt. He feels the need to accomplish their will before he can off himself. Their will's doesn't bind him physically. It's just that he can't bear the guilt of letting himself off easy by ending his life when his comrades died through horrible deaths.
I suppose he can't off himself yet since he wants to honor or respect the death of his comrades by completing their wills. It's a mental shackle( οΏ£-οΏ£)
MC can't die unless he completes all of the weird requests from his dead friends???