He was alleged to be the leader of Al-Qaeda in the Arabian peninsula. But, of course, he was a US citizen, and the drone strike happened in Yemen, a country we were not at war with. So it raised a significant number of ethical and procedural questions. Also, we killed his 16-year-old son (who was also a US citizen) with a drone strike several days later, also in Yemen.
but I don’t think your example is comparable.
Well, that’s the thing. Precedent is a tricky mistress. Sure, Obama had what he considered very good reasons for crossing that line, but it set a precedent that any subsequent president could follow. It’s like how George Washington set the precedent for presidential pardons by pardoning two men who were sentenced to be executed for protesting a tax on whiskey, and then a couple hundred years later, Trump was just straight up selling pardons to people for two million bucks a pop.
The point is, what seems reasonable when justified by a good president could easily be turned into something horrible by a bad president. The precedent set by Obama is probably not going to be as narrow as: “the US president is free to order the killing by drone strike of any US citizen who US intelligence agencies believe is a high ranking member in a terrorist organization (or a member of their family), as long as they are currently located in a middle eastern country”, just like the precedent set by Washington wasn’t: “The US president is free to pardon anybody who is accused of protesting a tax on whiskey”.