I also must say that even for the genre the dude downfall was pathetically fast
I really like how it made sense though. Given the explanations we got earlier it's plain to see - he went to the guild to say "I got rid of our (lowst ranked) trash member, I'm ready for our promotion to B-rank now" and they told him - "nuh-uh, the promotion offer was for your party as it were, with healer and all, without him we're going to have consider demoting you you D-rank"
(Then he gets pissed, overworks himself in attempts to prove he's worthy B-rank anyway, but has to spend all his money on potions and healing because it turns out that shit costs money when you ain't getting it for free.. which just serves to make him madder - and since he can't admit his culpability he blames the entire world for it)