Took me a second to realize who you meant by "they", since I was referring to the mercenaries by that term while you used it for MC's party without explicitly stating so.
Yes, to your great disappointment, I am indeed against murderers running rampant.
No, I'm not disappointed, I think you're being dumb and emotional.
This mercenary group isn't the only out there, what do you think is going to happen once it gets out that they were killed for revenge AFTER they had surrendered?
Lunette is also hurting for manpower after they'd lost their main general and had to retake the capital city.
You'd rather be Lawful Stupid and kill off a company sized group of mercenaries and the 4 other girls who are worth at least another company each, because "MURDERERS BAD", instead of accepting their service and working them to the bone.
It's not personal, it's business, and the business this time is Lunette needs able bodies across the entire frontline, because Touya is not going to be able to be everywhere at once.
If you don't get that then you're no different than the guys who get themselves and their squad killed because of emotions.