While we can debate the effectiveness or 'legality' of drone strikes why is this case any special? He's a member of a non-nation state militant organization, he's clearly (as much as this can be in such cases, but ISIS likes to take credit and not to hide it's footprint) responsible for multiple acts of violence which resulted in deaths. What does it matter if it's an extremist cleric who recruits, incites violence and supports ISIS, AQ, or any other similar organizations through video tapes from a cave or a "hacker" which uses twitter to do the same?