I think you fail to understand that the MC isn't a good guy, he doesn't need to be a good guy in this world.
Neither is he a bad guy though, he is just a guy who wants to survive as a seeker and oh boy it's a dog eat dog world there. He can't be naive and let people get away with things because...
Probably came free, nice cost cutting measure and I'd imagine they would have of course brought more than just apprentice potions even though in general in this world potions are not regarded highly in potency and are expensive (which is even more reason to take the gamble in testing apprentice...
As we know now Ambition is just how hard they chase their goals, in this case Braham's goal is to be the strongest and his ambition is so much he is willing to betray his own side at the drop of a hat to become stronger so an 88 ambition absolutely makes sense for him.
Nowadays MMOs have systems in place to stop it from happening but one of the tricks is to get a bunch of aggroable mobs and then port out in aggro range of other players, another is dragging mobs with aoe to do big damage on people you pass through or run the mobs through a group using aoe...
Monster PK is the act of using a train of monsters to kill a player which is a bannable offence in MMOs since its counted as griefing, ganking in a pvp zone is totally fine but that wasn't what the PK'ers were doing.