It is called treason and we have laws to deal with it. Killed in action in Iraq or Afghanastan ...ok. We crossed a border into a country we're not engaged with.
This whole matter makes me very nervous. Long-term, keep in mind who they are classifying as 'terrorists' lately.
It is far more than simple treason. He was waging war. That has moral and legal implications far beyond some domestic traitor who can be put on trial (though unfortunately we don’t do much about traitors anymore).
A traitor is not necessarily an enemy combatant as this guy was (a traitor can be a spy etc.). Capturing him was not any serious option, nor was it worth risking a single American life any more than necessary. I think the SEAL raid on Osama’s hideout was justified for the reason that we wanted to confirm identity, but in this case it was worthwhile to kill a bunch of jihadists and let the locals sort it out.