The passengers reported that the hijackers claimed they had a bomb.
With that in mind, they also knew of the attacks on the WTC. It is feesable that they decided that the aircraft should be destroyed before it could be used as a murder weapon against thousands.
It's reported that two seperate phone callers from that plane reported that they were going to try to retake it.
If this is true, Bless them. No Medal of Courage is worthy of their actions and sacrifice.