From the pictures on Facebook there was no sign of a struggle, the woman in the cell next to him and the cop monitoring the cells both told the same story. It is unlikely that the Thai police would suicide someone for drug crimes that were committed outside the Kingdom. There is just no angle in it.

The Thai Police were looking at huge publicity for arresting him. They would want him alive. Clearly he didn't have the connections he needed to be a rich criminal in the Kingdom otherwise he'd have evaded arrest (they had him under surveillance for a while.)

I think it is likely that he chose suicide on his own terms over a lifetime sentence in a US jail. Given the lifestyle he was used to, facing a lifetime in US jail would be pretty depressing.