part of the problem with the assasin class is it is a prestige class prestige class were supposed to be tied to the champaign world and heavily subject to dm fiat but insteade pepole me included just treat them like classes with prequisites picked and choosen for mechanical abbilities almost never built into the setting in any meanigful way. So like has been said you werent just a killer you were part of the order of assasins and shure once they have taught you their super special killing techniques you could leave but then youve just left the order of master killers and now must fight against them or whatever.