Should they get a court hearing? Probably. Is that practical, in some cases, if the person could be apprehended. Otherwise it would just be a pony show with a predetermined outcome, making no difference.
Sometimes, they kill citizens and then effectively say shit happens.
i.e. China sending a drone and blowing up a US citizens car, killing him and his family, somewhere in the middle of US and just saying that they were terrorists and a threat to China's security; but not not providing any evidence as that is classified.
Perhaps it would have been better in the specific case to first revoke the person's citizenship?
Revoking citizenship or at least leaving persons stateless is not allowed under UN-Charta afaik. Even then, would you blatantly kill (or better, murder) people? That's not rule of law, but rule of jungle. And as uncivilized as it gets.
I don't see a way to capture and try such an individual without having more citizens put in harm's way, or killed too. If the individual is in a training camp that is targeted and happens to be killed as well - I personally don't think anything of it. But that is starkly different than assassinating some individual in their apartment or hotel, or whatever. To me, one is warfare, and the other is murder - I do make that distinction.
Perhaps we must agree to disagree.