He won't steal just because he can, he occasionally gives any extra to those who need it, if someone is being a jerk or deserves to be stolen from, he'll jack everything from the house. He looks down on thieves who steal just because they want to. He loves that sensation of pulling off something big (my OB thief nearly creamed his pants when he stole the Elder Scroll). He might lift something from a wealthy merchant, who's only care is gold, then leave the payload on the step of a struggling family.
So all in all, he does it mostly for the sport, occasionally to teach a lesson, and sometimes when he gets bored. He's not evil though, he'll help out a merchant in trouble, instead of watching from the bushes and "acquiring" his wares.
I also have an assassin based on Artemis Entreri, those of you who know who he is will know what my assassin is like.
I have played a playthrough where I powergame, but I usually do that on FPSs, not RPGs.
So, does your character live by a set of rules/morals/obsessions?