I didn't dislike vaughn or the dialogue as much as some of you guys, but the finale made no sense. They fought and fought, and killed nearly everyone. They went and got the money to start a new life. Then both basically committed suicide at the end. It just made no sense.
Even assuming he goes to check on the kid because he can't help himself, Farrell gets in the car and drives, knowing its got a tracker on it. Why? He knows how to steal a car. He even knew no one was watching him. Then, after he manages to kill half of the pursuers in the woods, he inexplicably leaps out from behind a tree in front of 3 guys with machine guns. Why would he do that. He would keep moving from tree to tree trying to pick them off one by one, and he would succeed or fail. Instead he jumps out and commits suicide.
Vince Vaughan does the same thing. First, there is absolutely no explanation for how the mexicans found him on a random highway where no one knew he would be, since he just came from the cabin. Second, after he seemingly convinces them to take all the money and leave, he attacks one of them over his suit, with 5 guys already pointing guns at him. Why would he do that?
They just wanted to get to where the women lived and the men died, and didn't really put enough thought into how to get there.