I think most smokers these days would be aware that good manners require that they should offer to go outside in order to smoke. It has always been normal good manners to ask permission before lighting up in someone's house.

Of course the concept of good manners isn't universally understood...
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But I think Twain's aphorism isn't intended to be taken as literally true, inviting us to set out to try to weigh a range of human evils in some metaphysical scales, but rather as expressing a truth about the soul-destroying quality of lack of hope, despair, which closes down the future - and of course is a major cause of stuff like drug abuse.

Kipling expressed something similar in his poem Dayspring Mishandled.