Obviously that's not possible. But people are still missing the point so I'll try and be very clear.
The problem is that the UK and US governments are making the rules up as they go along.
A government operate for its citizens, so when it hands out punishment it has to be legal and accountable. In this case this guy (a Brit) got no trial, no way to plead his case, and was punished by death (which was abolished for murder in the 1960s). The intelligence is secret, and the decision to kill him was made in secret.
Few people are making a fuss because of his high-profile murders, and the media circus. But what about handing this justice out to other murderers on the run? Should the army just use drone strikes on any most-wanted criminal? Or is it only if you behead someone?
Maybe this was the only way to deal with him, but at least make it clear what the rules are, or it's extrajudicial. The alternative is slipping further into the world of Fahreheit 451, Nineteen Eighty-Four, and Minority Report.