Imagine you're there, in the arena, adrenaline practically oozing out of your pores. You're 500 feet above the one of the most dangerous cities in the world [1], a warzone that's claimed the lives of thousands of young men, just like you, and you've just taken out the armed enemy. That was the enemy. And that guy laying there still is the enemy. And so is anyone helping him. And now a van has pulled up, unidentified. You don't know who or what is in that van and whether it is armed or not. You know the reality,the statistics, and they're not in your favor. So you do as anyone in that situation would do, with limited information and a hell of a lot of empirical data. You neutralize the threat. You destroy them and live to see another day.
[1] 2007 was the bloodiest year to date in Iraq - http://icasualties.org/