Did Sam whack him as Paul said about "you can never go too far" and Sam though Paul was becoming too pushy?
Whacked Tom Cruise http://good-times.webshots.com/photo/2196812000100119259ULHDOv
It's not a single gun shot. Rather, there were 5 gun shots - a bullet for each one of Hau's family member. Sam was relying on his Thai friends' assistance. How could he possibly whack Paul when the latter was the one calling the shots?
"You can never go too far". This could be translated as, there's nothing that is considered too inhumane or extreme. Getting rid of the entire Hau's family is not a big deal; at least, that's how Paul puts it to Sam.
It was tying up loose ends. Sam didn't wanna go that far, but the Thai dude prolly felt it needed to be done 1) to commit Sam to their partnership, and 2) to prevent the Ngai family from seeking revenge (since they are all dead).
shareI agree with holyabdul it seemed like that there was no other option
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