Is what results from a person harming himself, such as illnesses or injuries from smoking, part of Allah's divine decree (qadaa’ and qadar), and would he have inevitably suffered from it, or is it that the person himself threw himself into destruction?

Everything that occurs in existence is by the decree and preordainment of Allah. Allah has decreed its occurrence through causes that necessitate it, so both the cause and the result are by the decree of Allah. Thus, the smoking of the person that caused their illness is by the decree of Allah, and the person has no excuse against Allah, for Allah has established the proofs and removed the pretexts. Allah knows what was before it came to be, and He has written it down, but Allah’s knowledge does not preclude praise and blame, reward and punishment. As for what would have happened if the person had not smoked, knowledge of that is referred to Allah alone, for He is the one who knows what did not exist, how it would have been if it had existed. This person cast himself into destruction by his smoking, which is a forbidden cause of his illness, and he is held accountable for his action. It is not possible to state with certainty what would have afflicted him had he not smoked.