Not sure you're on target here, Mick. OBL in essence declared war on the United States. This he did both with his words and with his actions (saying killing Americans "anywhere, anytime" was a justifiable means to an end). Some will argue that once he took this tactic, he a battlefield soldier and not subject to common criminal code. In that sense, one could say that his killing was a responsive act of war, not an execution.

I'm not at all sure that killing him will be a better result for the USA than would have been taking him alive. But I get the logic behind what they did.

There also exists the possibility that they did intend to take him alive or dead, depending on circumstance. (That's what they said, anyhow). This seems perfectly plausible to me.