Abdulrahman al-Awlaki was not on a kill list. He was not even accused of a crime. He was clearly the "target" of the US drone strike. Even if he was "just hanging out with these high level terrorist" (which has not been confirmed) does not give Obama the right to target and kill a US citizen who has not been accused of a crime and without due process.
Furthermore,
the administration has neither acknowledged his death or acknowledged that it killed him. It has, indeed, done everything it possibly can to avoid saying how and why it killed him, and has answered the Freedom of Information Act lawsuit brought by the ACLU with a blanket insistence that it is not obligated to confirm or deny the existence of the CIA's drone program, much less disclose information about those the drone program has killed.
http://www.esquire.com/blogs/politics/abdulrahman-al-awlaki-death-10470891
What evidence? There should be full disclosure of who, how, and why anytime the US Government kills an American citizen! You are only assuming that is the most unlikely scenario.
The definition of "engaged" is simple in this context. Waco was engaged against the Federal Government. Randy Weaver was not engaged when CIA agent, Lon Horiuchi assassinated his wife at Ruby Ridge. Christopher Doner was engaged at the time the smoke grenade was tossed into the cabin he was in. Abdulrahman al-Awlaki was not engaged.
And, moreover, if you believe that some secret deliberation made behind closed doors by the Executive Branch counts for "due process" for American citizens that may or may not have even been accused of a crime, then again, we, at this point can agree to disagree.