1st. Why join them? They are demon hunters and MC loves their fucking princess..
2nd. Condition was to defeat 2 of them. It was him who attracted out of nowhere. Fighting experience won't help you with fucking betrayal.
3rd. Antagonizing great mage who is going to join you is retard move.
Why plot turned so stupid.... I loved this manga at some point, not it became your average 4/10 manga.