I thought that was the case too; but didn't the author show us his job life and how his coworker treated him and showed us his inner monologue? and I don't remember the monsters had the ability to disguise themselves as humans; if he was a monster in disguise the stuff the author that showed us about his life wouldn't make any sense; also my point still stands the characters didn't give a damn about him like I know the ML is a selfish prick but I thought at least the MC will say something about it.
I think MC why like this because of the experiment that was done on him; I think he was made to eat monsters rather than humans but maybe he is that way because of that alien who knows? and I don't think that guy is a monster except if he didn't realize it, but even then the MC knows himself that he is not normal he is eating monsters and drinking blood; but we didn't see that with the guy at all, all he was ranting about was how shitty his life is.
RIP the character who was there for 3 chapters then ended up dying and everyone acted like he was never there even the MC who is the "good guy", I don't know why he was there if the author was going to kill him and act like he never existed, the plot would be fine if the MC was alone on that hole I don't get why he was there he has zero role there.