Sven and his necromantic helper aren't above looting as said earlier. And the necromatic guy was asking Sven if they were going to replace the MC by injuring him enough (that healing potions/skills can't fix him up) so that he is able to adventure into the labyrinth. So it looks like Sven is basically semi-bad guy that chooses to work within an established guild because that provides more rewards than by being something else.