how could he think he was among friends with them standing over him about to kill him?
What kind of stupid idea is that?
You need to keep up with the threads. I take the father literally when he says his son was killed by friends. There is lots of evidence that the Berg household was, to put it euphemistically, "progressive".
I'm not convinced Nick was killed on the part of the tape that everyone has seen. The video has been clumsily edited, shot from at least two cameras with different clocks set to different times. Some say there is lots of blood on the floor, but no blood flows when his neck is severed. The sound track is also off sync.
My interpretation: He thought he was posing for a propaganda film and didn't know he would actually die. The film of the actual killing is edited out. There is no additional blood during the beheading. He doesn't struggle because he is already dead.
Further tinfoil thoughts: he thought he was in Iraq to help them, and trusted them. They stopped trusting him when he attracted the attention of the FBI.