Should I kill Scanthril behind the burnt out house? I talked to the leader and he gave me two quests. Should I do the quests or just kill the rats?

The quest to kill Raskilion will be required in order to have a characer become an assassin...keeping Scanthril alive until at least that long would be a good idea.
I believe that whoever talks to him & gets the quest from him is the one who will be the one who has qualified to ascend to assissin if that is your goal.