You're right keda. There is no Good Samaritan clause in Thailand. If you stop and administer first aid and the guy dies later on, the family can claim that it was the fault of the person who gave first aid.
I know some farang paramedics who work in the hospitals. They never stop at the scene of accidents and help out, for this very reason.
It's a tragic situation, but, it's just not worth the risk.
The first rule of a firstaider is "Don't put yourself in danger".