I thought that the original rule was that you can kill someone if an attack is imminent, and also assuming that you're not at war (which we're not). However, this rule has been subject to mission creep, particularly by the US, and they now see it as legal to kill someone who has attacked you in the past, or whom you think may be plotting.
It also obviously depends on the physical context. A drone strike in the Syrian desert is probably considered legal, providing that the there is sufficient evidence of an attack, past or future; it's less likely in a bed-sit in Basingstoke. Of course, if an attack is imminent in Basingstoke, then the main rule applies.
I suppose arguments arise with people who want to renounce IS, and want to come home. It seems a bit harsh to shoot them at Heathrow, as they get off the plane.