Need help with something.

And yet that impolite kidnapper was the one who was mostly taking care of Sean, against any idea of even killing him, wanted him to reunite with his parents etc, and yet when the father arrives, he was pointing a gun at him, rudely forcing him to give the money etc and even started a shootout with the FBI and killed one of its agents etc.

The point of Gary Sinise NOT giving Donnie Wahlberg any more instructions after getting the money from Mel Gibson was that Gary Sinise never planned on returning the kidnapped son. When Gibson sees the confusion in Wahlberg’s face, that leads him to the conclusion that the voice he’s been talking to on the phone (Sinise) is not to be believed, that he is never going to get his son returned by the kidnapper, and to turn the tables on him instead.

I don’t necessarily think Sinise was outright planning on turning on his own crew from the start, but rather was a natural progression of dealing with the problem of Gibson making things difficult. Funny though, that all the problems stemmed from Sinise not planning on returning the son from the start, because if he did everything probably would have gone smoothly as Gibson was at first taking the advice of the authorities of just doing everything the kidnappers say.