Our intrepid hero just watched as a group of rebels executed some of the tyrannical leader's soldiers. They're shocked! How could they do this? Don't they know that killing is what the tyrant does? The rebels laugh it off. It had to be done, they would've done the same to them

A More Civilized Age discussed this exact point recently; There’s a very valid reason for resistance fighters to not take prisoners. One, keeping prisoners requires resources you may not have. But more importantly, the goals of a resistance group and the goals of the occupier are not the same.

As a resistance group your goal is not total military victory and occupation, your goal is to make continuing the occupation as painful and expensive as possible, to convince the occupying force that continued occupation isn’t worth it. Taking prisoners directly goes against this goal, unless you plan on using those prisoners as some sort of bargaining chip. Especially in a sci-fi setting where wounds can be healed quite quickly and thoroughly and so the wounded can be back in action very quickly.

And YES! They would have done the same to you! Without hesitation! And they still might try if you don’t kill them!