you understand that in solving murder a motive is usually established first, in Thailand especially involving farangs that is usually were someone has something to gain and is almost always financial - I connected a very recent trip home to the possibility that he brought cash back with him which could very well be at his home and is also untraceable since it is not in a bank account, I know people that do this when visiting their home, the only person that knows about this cash is the deceased and very likely his live in Thai partner.
financial gain is a good place to start when looking for motive but very difficult to uncover were a wad of cash is concerned, his home bank might show a large withdrawal before returning to Thailand, this would indicate he had cash at his home, would Thai cops look at this - I'm not sure.
alternative to this would be a business disput (again financial) or mistaken identity, there's not much else that fits