Are governments, or even private individuals, allowed to kill terrorists when killing them is the only way to prevent a terrorist attack? Intuitively, I would say “yes”, but only if certain conditions are met: the attack must be imminent, and no other solution is possible. In fact, these conditions limit the possibility to cases such as killing a terrorist with explosives clearly visible, and seen – from a distance – to be moving towards a target.
Most cases will be different and will make it possible for the police or bystanders to disable the terrorist in some other way, short of killing him or her, and without putting themselves at risk. I never understood why the British SAS needed a policy to target and kill IRA terrorists when they were not engaged in an imminent terrorist attack and when they could easily be arrested (see here for the story).
Now, one could reply to this with this question: why should we treat terrorists better than soldiers? In a war, soldiers can be killed almost at will. If an army spots enemy soldiers, it can kill them without violating any law of war, even if these enemy soldiers are not engaged in an imminent attack. So why can’t we kill terrorists in the same way? In fact, we should treat soldiers better, since many of them are conscripts who do not target innocent civilians. Terrorists are (normally) volunteers who target innocent civilians. That makes two aggravating circumstances.
In answer to this, we could state that terrorism isn’t a war; it’s a criminal act. Some things are allowed in a war which aren’t allowed in peacetime. And terrorism is horrible and not peaceful at all, but not everything that is horrible or a breach of peace is necessarily a war. If we are allowed to stop the crime of terrorism with targeted killings – even if the crime is not imminent – then why not normal murder as well? For example, we may know that someone is about to commit (a non-terrorist) murder, but the act is not imminent. If you accept the SAS tactic, you also have to accept the preventive killing of normal murderers.
Some go even further, and accept not only targeted killing in cases without an imminent threat, but also killing after the fact. They would accept the killing of Osama bin Laden, even if he wasn’t planning a non-imminent attack. They would justify this killing based on his past actions. (Another example is the targeted killing by Mossad of the people involved in the Munich Olympics killings, made into a movie by Spielberg). I think that’s just as unacceptable as the targeted killing SAS style. It’s punishment without due process.