I am amazed that someone smart enough to be a chess grandmaster is apparently not smart enough to "Just say no" to drugs.
Or maybe he didn't take them voluntarily, and his death was foul play (a wild claim which is entirely speculation on my part, based on no evidence whatsoever).
Some people just like drugs. Lots of people engage in risky behavior because they like it and are willing to take the risk. Look at basejumpers for instance.