Pose no immediate threat to who? To the government doing the killing? True (but see Paris and Brussels for counterexamples).
Pose no immediate threat to humans? Absolutely they pose a threat.
The drone powered war is a system in which an organization on the other end of a robot is acting as judge, jury, and executioner, with no way to appeal for recourse or even open a dialog, and this organization also has a history of extreme inaccuracy (which they try to hide under the guise of national security).
I see drone usage as just as reprehensible as the terrorism attacks carried out in Paris and Brussels, in that in both cases innocent civilians are dying and afraid of dying.
Other than that Belgium and France need to fix their shit and have less people falling for crazy extremists.
This isn't new. Paris and Belgium has a history of at least a century of terrorism.
With those bombs... look.. if that guy was a terrorists, how many more do you think it created? Dozens, maybe hundreds!
And that guy would still be an alive threat.
Imagine an ordinary American citizen, maybe not mentally stable (anymore), maybe living in poverty, seeing how Japan for example would have drones over the US, seemingly randomly killing friends and family. Imagine there was a group of people, calling themselves Patriotism for America. Also there have been wars ravaging the country, everything is ash and dust. There is more weapons than everything else. Imagine some people there had contacts and friends in Japan.
Guess what would happen.
Hint: There were way lesser reasons for many wars and acts of violence.