Presidents have been assassinating targets overseas who pose a threat to the US for many many years. It's just that Obama was far more open about it. Trump carries them out all the time. Why the double standard? Why do you only hold Obama to your very high standard?
Because Anwar was a US Citizen.
Emphasis on "was." He effectively renounced that citizenship--and thereby any of its subsequent rights/protections, such as due process--when he actively allied himself with a declared enemy of the US.
He should have been convicted of treason, then.
Aside from that, however, is the fact that police officers routinely deny US citizens' their right of due process every time they shoot to kill a suspected criminal. The very act of authorizing police officers--as a general operating principle--to use deadly force against US citizens is, arguably, a defacto suspension of the right of due process, left up to the individual officers to determine in any given scenario.
The DoD doesn't need that blanket authorisation, though. Not for assassinations.