Ok here is one I guess
Why are mercenaries maligned in fiction?
In terms of being an asset mercenaries were often much more skilled, knowledgeable, and often equipped than long standing armies.
This goes even further where some orders who were Mercenaries (Hello Knight Templars!) at least for a time are... sort of sanitized of that aspect.
They are often fairly traitorous with no real loyalty to anyone. Which I guess is because... as Mercenaries they have to play both sides after their contracts are up.
They are often quite criminal, basically acting more like Bandits who are being legitimized.
1) They are easier to vilify. There is no mercenary draft, nor a higher mercenary cause. If your mook is a fighter who is not a mercenary and you have your hero stab him in the face until he dies, your audience might wonder 'Hey, what if that guy was just some mostly innocent schmuck who got drafted by/owes fealty to/was brainwashed by the wrong side purely by an accident of history?'.
2) Even being on
your side, they don't particularly care about your goals. At
best, they will just peacefully fuck off when the money dries up. And since your side is clearly The Good One, that makes them Not So Good.
3) ...they
were, historically, both traitorous and quite 'criminal'. The latter is, well, the norm of war for most of history even for non-mercs, the former because, well... if you paid some guys to fight for you and they are a significant part of your manpower, they might start to wonder if they wouldn't be better off fighting for
themselves, doubly so if you stop paying them.