So it would've been really easy to prove the case in court. That's not an excuse not to have a trial. They knew where he was (to drone strike him), they could've tried to arrest him. Hell, there's even the concept of trial in absentia. Killing anybody and saying that the President's say-so is all the process that due is exactly what kings do, and what the American Revolution was about.
And, yes, I do have a problem with the way that many other cases were handled. We're well down the slippy slope to a place where the man in charge can blow up boats in the Caribbean without evidence or trial, solely on his (dubious) claim that they're drug-runners.
So, fuck-you very much, Obama.