Falling from a head position in a communist government to being placed under house arrest while basically waiting for your death sentence can break a man. After having so much power and hurting so many people it becomes difficult to stay blind to the harm you have done once that power is removed. When given no more option other than contemplation of his past, I think he realized that his death was the last thing he had control over.
To me it would seem the more fitting questions would be rather Pol Pot died from a heart attack or if he did indeed commit suicide, seeing that the later was never actually confirmed since he was cremated right away. Given that he was going to be handed over to an international tribunal I have to imagine suicide is the most likely.
I do not think that any one killed him secretly, all of the evidence pointed to him taking his own life, and that is what I think he did. I do not think that some secret killer could have gotten away with it and not been found. This was all him.
I believe Pol Pot committed suicide. I don't think there is a reason to believe he was killed by others because drugs were reportedly found nearby. World leaders have done this before. Hitler is the best example of this. Pol Pot's role at the time and his actions in the eyes of the world caused him to commit suicide.
When you read about the life and thoughts of Pol Pot it seems an impossibility that a man of such ego and arrogance would kill himself. He genuinely believed he stood for a good cause and was slowly making the world a better place. I don't think his atrocities ever haunted him.