I think the thing about it is that Wario isn't heartless, he's a greedy jerk, but he has a heart of gold to some extent. If that wasn't the case, I doubt he'd have any love for a pet like his hen to a point he would drop his quest for a moment in...
As everyone has said, greed is definitely his most defining thing first and foremost, but Wario being so driven is something that defines him to me. He gets his ass beat in SML2 and decides that if he can't steal a castle... he might as well go...