Absolutely a world of difference between armed combat and first-degree murder. In combat a man has shown hostile intentions, and has the ready capacity and intent to do you harm.
That is completely different than entering a man's bedroom and killing him. The devils are in the details however. If he was appeared to be grabbing for a weapon, then he would be posing a threat. If he grabbed one of his wives to use as a hostage, that would be hostile action. If he had was wearing an explosive vest, or exhibited any behavior that the Seals though was aggressive behavior, then they could shoot him as an enemy combatant.
But if the man's name is Al Capone or Hitler, and he is just standing in front of you, you can't murder him. You can take control of the situation and hand-cuff him, and take him prisoner. If he makes any action that is life-threatening to you (like pulling a knife from his boot), then you can shoot him to protect yourself.
I was in the Navy, and I know the training. I would be really surprised if those Navy Seals shot OBL in cold blood. They are rigorously trained on how to handle every situation - and I really don't think any one of them could have simply shot OBL without cause. It is not in their blood.
Hey, did you see the latest notepad they found in bin Laden's house?