@Antairan
I really don't get some authors, they try to portrait a vengeful character but end up being dumb saints as well as wasteful people.
Hajime isn't one of those vengeful MCs that try to get revenge at all though; after he fell and transformed due to the monster meat, he didn't give two shits about his classmates, much less getting revenge on anyone. He struggled only for survival. Sure, he'll kill people and get revenge on them if they cross him now, but he doesn't care about what happened before the fall at all really.
That is also why he didn't let Shimizu live here. Shimizu was already too-far gone and would probably plot a revenge if left alive, not to mention that he was pierced through his stomach with a certain fatal wound. There was no point in healing him when he'd just cause further trouble down the line, so Hajime shot him. There's also a few other reasons he shot him instead of letting him bleed out, but I'm going to let the next chapter (hopefully) explain some of those.