Massacre has no implication that people were not fighting back.
A 13:1 KD is still extreme, especially when it is often reported that those killed were non-combatants.
To jump the gun a little, yes I am well aware that during insurgent occupation it is impossible to distinguish between combatants and non-combatants.
There are very difficult choices to be made too - if locals are not showing you who the terrorists are, they're enemies. Irrespective of the threat they are no doubt under from these terrorists.
As soon as you are a fighting force hiding amongst the populace you are no longer part of a legal army, you are a terrorist, and anyone assisting these people are also the same.
If you're getting 13:1, you're killing huge numbers of people very easily, be that because of better training or because they are unarmed enemies. That still makes it a massacre.