What the? The holy knight also fell into the water and it doesn't even get addressed how he joined up with the others, still fully clad in armor that would have drowned him, not even wet, and suddenly fights together when his priority was and should be to kill the priestess?
He has no motive to kill the boss or clear the dungeon. As soon as the priestess died, he could turn around and leave.
But well, it didn't make sense for him to clear all the trials and catch up to the party in the first place with the headstart and the best guide possible they had.