The idiot Killer Frost started it. Once she found out about the helicopter she pushed Boomerang and Deadshot onto the floor while she ran off. This triggered the "every man for himself" mentality in the two of them, so in the end everyone had a pointless rat race to the helicopter, when they could've shared it.
Deadshot could have driven it, while Killer Frost and Boomerang could have shot down people who tried to take the helicopter down.

Every man for himself means you screw over the other guy when you actually have something to gain by doing so. By screwing each other over just for beep and giggles, all they did was increase the chances that NEITHER of them reached it.

Because its every man for themself.
Yes but still, it's smarter to share the chopper and once there they can be dropped at different locations. I may not get along with someone or even hate them, but if am running away from something more dangerous and powerful I would be smart and share the helicopter with them instead of fighting with them and waste valuable time. It makes no sense, I know it's a cartoon, but this part of the movie made the story lose credibility.

Because its every man for themself.
Lose the Game!!!!!!!

because bad guys run solo

well considering their situation id say they were all pretty desperate and panicked to escape Arkham. Plus, Deadshot & captain boomerang had been fighting throughout the movie, those two couldn't get alongso of course they just HAD to pick that moment to escape to use to fight each other instead.