I've said as much before, but haven't the US been trying to kill him for a decade already. Spy drones, air strikes etc. Yes, they went in and eliminated him, that much seems obvious, and why then did they pretend it's something else. They couldn't put him on trial, a closed trial would have no legitimacy at all given their relationship, and an open trial would be a platform for him to preach from, and besides the US doesn't subscribe to the notion of international courts.
If the official stance had been, 'yes we killed him, it was a military operation in an ongoing conflict'. I don't think there would have been much in the way of outcry at all. If we can't class him as an 'enemy combatant', then who, honestly.