The OP states the guy sustained severe head injuries.

I don't know if that is possible if one is wearing a proper helmet.

While the truck driver is a coward, it is possible the deceased contributed to his own death.

2 hours ago, London Lowf said:

I think that dying as a result of a road traffic accident is classified as "natural causes" in Thailand.

 

Is it any less natural than succumbing to cancer after smoking, or cardio-vasular disease after a lifetime of junk food?

 

 

 

I believe you might be right, the traffic statistics here are so horrendous, that the authorities likely do everything in their power to fudge the numbers. I don't think the the death toll is anywhere near the real number. 

 

When I lived on Samui I was told to buy a friend who was on Samui Rescue and spoke fluent Thai, that the authorities dramatically reduced the number of fatalities reported each month.