@kirin-kun
I think it's you who live in a "perfect world" "cute world"
"people being killed by the MCs deserved death"
it's your interpretation, with what you've read, but the MC has real information before him, you only have a text and a drawing and your biased mentality
biased because you live in the 21st century in a world without magic and with your laws, moral and custom
"had no family to support and nobody will ever ask about how they died and why"
bandit is not a job, he does not come home at night or once a month to see his family and talk about work
bandits are people who have chosen to be outlaws for ease
they probably have no family, or been abandoned young
Bandits are terrorist (bandits in groups who murders and rapes)
it is only in our world, in some countries, that terrorists are lucky enough to be in prison, or even free
the only point on which I agree with you is the part "Adventurers are handed quests sanctioned by a guild normally" but even here it is only supposition
you can very well be a mercenary (in addition to adventurer) and declare your actions as mercenaries and pay taxes on them
the girl who gives her the job can easily be considered the authority on the spot and replace the mayor to make a request to exterminate bandits, besides she paid with local tax money.
legally it is wobbly, but for the time it is largely enough, besides who will come to check the legality and especially how?
even with an official stamp of the mayor, it would change what
he would send a request to the guild or the lord, they would not have investigated, they would just send people to kill them
I remind you something, they kill a village of 100 people, no need to investigate it is necessarily terrorists
it's not always the case, it's just the norm and a simple way to explain complicated things
a world that gives work on paper more expensive than the reward that is written on it, almost everyone can read
the same organization works in several countries and these workers can go where they want
all these things and others are not developed in a manga and it's not for nothing, it's complicate and it does not interest most people.
But sometimes authors have interesting and logical systems in their WN or LN, but this is rarely described in the manga version
your way of thinking reminds me in "Potion Danomi de Ikinobimasu!"
when the translator and a lot of readers talked about the Geneva convention
as if being a person reincarnated from Japan made you someone who will respect this convention
which does not exist in this other world
which moreover has a development much less advanced than us at the time of the ratification of the "laws" concerned
"laws" that are not even respected by all the countries that signed them