This party accepted a request from a ghost to avenge her rape (and her family's murder), by killing the bandit perpetrators.
Each society defines its Justice by the law. It looks pretty obvious that these bandits are not acting within the law, so they are criminals. Period. And should be punished by the law enforcement, after due process.
So, this Adventurers party is acting like vigilantes, taking Justice into their own hands. So, they are acting like hired thugs.
Maybe, in this world, Adventurers are given the right to administer Justice as they see fit. Which is problematic in itself, for all kind of reasons. Mainly because of malicious denunciations.
In some isekais, people doing bad deeds are "marked" (with a status, or some aura), which is more convenient.
Most cultures, even if the laws are different, have a system to handle the "criminals". Here, the uncomfortable feeling stems from the fact that the adventurers are judge, jury and executioners. And even paid for it. Just for the purpose of freeing their friend from possession.
Ie, they are morally bankrupt too.