Fighting against invaders, that isn't terrorism, and nor is fighting against insurgents. It's just a very unpleasant type of warfare. But deliberately targetting civilians, and killing prisoners, that is terrorism.

The one spills over into the other, but there is a distinction. Terrorist acts such as those are war crimes, whoever does them. It would be perfectly appropriate for the people who do those things to be tried for it in a War Crimes tribunal. It would not be appropriate for the same to be done for people who had taken part in irregular warfare, without resorting to terrorism.