Why would you call it a sin?
Well, sorry as I am to see people smoking and putting stuff into their lungs which they could avoid, it is not against the law, human law, so it is simply a choice you make - a choice that is probably detrimental to your health, but which you have the right to make.
Ah, just saying about the body being a 'temple' which we're not supposed to defile. No, smoking kind of isn't against human law but where we can smoke is being limited every year, just so other folk don't get it in their lungs through second-hand smoke or even just a whiff of it.
Yes, possibly detrimental to the health...a choice to practice...a smoker's right to treat themselves as they see fit. Sin?...
...depends on one's definition of 'bad'.