I'll see if I can find a link to back this up but I remember reading an article about this at the time. The mission wasn't approved by Pakistan, it was a top secret incursion into a nation that might have responded with force had they known. That meant they were on the clock. A fast in and out mission was essential to it's success.
One of the choppers went down, making getting in and out fast all the more difficult.
The orders were "Take him alive if he'd come quietly", the Marine, when he saw Bin Laden said "surrender" or "lay down", presumably in Arabic. Bin Laden didn't comply, so he shot him right then and there. If he'd laid down, they might have taken him alive.
Given the necessity for speed, capture alive wasn't the priority, but it was the plan if he'd not resisted. That's what I read anyway shortly after the event. It's possible this was just words to make it sound better and not how it actually went down, but that's what I read.