Even if they don't realize it, the time that they've already spent sucking on burnt things, the change in their habits, and the effect it will have on the rest of their lives is just so amazingly devastating.
Screw the health issue. We eat stuff that will kill us, we do activities that are unhealthy - a lot of things in the world threaten our health a whole lot. But smokers alter their entire social life to hang out with other people for the sake of smoking, actively disgust and discourage their existing friends, and become amazingly less personal due to the fact that they have to remove themselves from their daily lives to go and feed an addiction that doesn't help them in the slightest, while funding an evil industry just because they want to be "edgy" or something of the sort.
What's more, the longer they continue to smoke, the less likely it is that they'll be able to quit - and that will affect their lives. It will affect their kids' lives - and maybe that's where its unhealthiness hits hardest. It saddens me so much to see the start of a negative life decision, but I can only imagine how horrible it will be down the road.
Seriously, I think what pisses me off the most about cigarettes is that they are so amazingly unhelpful and unnecessary. That fact alone makes me lose faith in those who I used to call friends simply because they're letting themselves be consumed by a void. I can't think of them the same way - and neither can a whole lot of people - simply because they've decided to take part in something so obviously harmful and useless.
My room faces the quad where people go to waste their lives in the ash tray and I want to replace butts with something more striking, like "STOP SMOKING." Unfortunately that wouldn't fit very well without hyphenating it, so I'm thinking of something more direct, like "FUCK YOU."