It's all Maggie Grace's fault!
From a story perspective if Maggie Grace hadn't resisted capture then everything could have turned out OK. The truck wouldn't have crashed, the groups wouldn't have got seperated, Nick wouldn't have run off and killed Ennis, and got killed in return.
The balance of power shifted twice between the two groups after they first met, with one group taking the other prisoner and then the other way around. But we knew they weren't going to kill each other because they're all good guys. If Maggie Grace hadn't resisted then they would have eventually figured out they're both on the same team and started working together.
Just a thought...