If you looked closely, the flying bug has poison attack on top of paralyze, and one dude who took it head on got their hp halved and that's before poison debuff ticking. It is very likely he got poisoned to death before anyone realized.
Also I think the other priest either can't heal or can't cure status effect(s) and focusing on buff build, not sure though since the game skill acquisition is quite extreme, allowing low levels to gain access to staple aoe heals/attacks without investing on lower skills.
Anyway it's been 26 chapters and there was no sign of mc over focus for self-inserters like AW and SAO, I think Reki is improving in his storytelling, he's always been good in world building so if he (hopefully) keep this up, Demon Crest is going to be his masterpiece.