Personally, I would prefer not to stab anything. But, I will take a shot at it.

Karma in sanskrit means action. And, you don't have to take my word for it, sensei noded it. You acted upon a request in a way that was giving towards a fellow human being. So what if it was a cigarette? When it comes to bumming out smokes, I like to think of it this way: smoking is a stress reliever. Stressed out people do stupid and dangerous things like crawling up the clock tower and picking off citizens with a shotgun. So, in the long run, by giving someone a cigarette, although it may contribute to their death, might save countless other lives.