I think it's safe to say that the burning of something that you directly inhale is the principal killer, regardless of what's in it. When burning organic matter, there tend to be many byproducts produced. Often more harmful than not. Odds are we didn't evolve to breathe them on a constant basis. If we had, this stuff wouldn't give us mouth and lung diseases so much, would it?

If we had evolved to tolerate doing this to ourselves, then we wouldn't be arguing about this. IMHO, some people do seem to adapt better to burning things and inhaling them. BUT I'm not going to test that out on myself for 20 years only come out the other side with lung cancer (or worse). It's a crapshoot, really. Your call. Plus, I like the fact my sense of taste and smell works better than it did when I was a smoker.

If the nicotine in tobacco didn't give you the high you feel when smoking, would people still be smoking and getting cancer? Probably not. People like getting high. Take that away and what reason is there to smoke? But I digress...

So, to answer your question, "all of the above". You smoke to get high from the nicotine, but the other shit in the smoke is what your body can't tolerate (generally speaking, as yes I agree there are many instances of people living perfectly normal lives smoking the whole time).

I'd take up smoking e-cigarettes if you feel it's safer to you (long term) than smoking cigarettes.