Within reason of course, there need to be a control layer of experienced judgment. If three out of three randomly drafted anonymous judges are of the opinion that a target (mob boss or terrorist mastermind) must be erased in order to protect society against deaths or loss of property sure why not. Drone killings and high tech assassinations to destroy vermin are just too convenient and cheap no to use. Mark my words they have come to stay.
Yes, targeted killings can be justified. All Governments can make arguments that will justify a targeted killing. In the case of the United States, the targets are known terrorist leaders. If left alone, these individuals would cause death and destruction on a very large scale. It is a sad commentary on today’s way of life, but there are times when targeted killings are necessary to save thousands.
Targeted killings are difficult to support in a democracy, but there are extreme cases where it is necessary and justified. Osama bin Laden would be an obvious example. He was guilty of terrorism and mass murder and was actively planning future attacks. Few would argue that his targeted killing was not justified.
If there is somebody that has or undeniably is working on killing groups of innocent people then yes, deadly force to stop them makes sense. Those that have no regard for human life should not expect their own to be valued. It needs to be limited to these cases, though. It also shouldn't be glorified like Israel made it recently with their boastful uploading of a vehicle being blown up.
While justifying the targeted killing of a person or persons known to be a threat to society is understandable, it is a dangerously slippery slope. The definition of a threat can be a moving target – is a threat someone who is planning a mass homicide? Someone who could be planning it? Someone who is a threat to societal institutions? At what point does ideology become threatening? One could argue that strict standards could be created to eliminate definition justification “creep,” but standards can and do change. It is naïve to believe that targeted killings do not happen, but they should remain in the gray to dark area, and should not become a well used tool of defense (or offense).