First off,i don't smoke in real life. I can't stand the smell of nicotine let alone smoking a fag myself...
Diandre suffers from schizophrenia . He finds smoking as a means to calm his mind and seek refuge of reality and find his fantastic utopia. Like many other known writers,this kind of practice is anything but uncommon. Charle Baudelaire,known for being inspired from his depression, claims to seek his artificial utopia through the means of alcohol (wine to be exact). Paul Verlaine and his lover Arthur Rimbaud found inspiration for their brilliant work by drinking absinthe and smoking hashish (marijuana). Literature and arts are the best emotional representations, artists go through suicidal and depressive phases solely to illustrate them through their work for us to understand and see. We are all flawed, and what we do with our flaws matters.
My point being that...Diandre is not perfect,he's very aware of that .He's sociopathic and lacks genuine compassion for people around him (At least, that's what he tells himself. ) The only way to escape from his difficult situation is by smoking , it clears his mind from all troubles and helps him seek his artificial utopia.
Understand however, that you're entitled to disagree with his coping mechanism ,that's completely fine. But at the very least, sympathize with his flaws .