This is in part because sovereign states can and do negotiate their "terms" (i.e., for peace) and they typically extend recognition and "protections" to each other's combatants. These states then are not by custom, convention, or treaty obliged to extend to "terrorists" or "Illegal Combatants" that same recognition--nor to BRIGANDS (look it up) as they would to enemy PoWs.
ILLEGAL is a strong term that nobody really likes to use but it it what it is.
Remember the example I gave you of Peyton Farquhar, the Confederate sympathizer and plantation owner from the famous Ambrose Bierce story, An Occurrence at Owl Creek Bridge. If Mr. Farquhar had been wearing a Confederate uniform or simply identified himself as a Confederate soldier under color of law, then he would (or should) have been taken prisoner-of-war instead of hanged.
The ICRC provided commentary saying that "regular armed forces" satisfy four Hague Conventions (1899 and 1907) (Hague IV) conditions. In other words, "regular forces" must satisfy the following criteria:
─ being commanded by a person responsible for his subordinates to a party of conflict
─ having a fixed distinctive emblem recognizable at a distance
─ carrying arms openly
─ conducting operations in accordance with the laws and customs of war
PLEASE NOTE THAT "Illegal" and "Irregular" are not the same thing, and I am not saying that; they are related terms but you have to be very careful with the terminology if you are not clear about the concepts. Guerillas are probably Irregulars and they may or may not be legitimate or legal.
This should all be pretty clear from the two articles that I cited, especially the second one on Irregular Warfare.
I am more than willing to continue trying to explain this or to provide more authoritative evidence if you are are legitimately interested. I wish that Junior Bush had understood the concept better.
"Unconditional Surrender" is another War Crime--or at least another egregious atrocity--but that may be another separate issue than what we are addressing.
In my opinion, Churchill's plans to "Set Europe Ablaze" was a huge War Crime or egregious atrocity, but Winnie did not lose the war so was never held to account in that regard.