A lot of times, Geralt's monster slaying in The Witcher is equated to a moral judgment.

Here's a hypothetical quest:

Bill drowns his wife in a river by the village. Unbeknownst to Bill or any of the villagers, Bill's wife comes back as some kind of ghost and, blinded by rage at her death, lashes out randomly against anyone coming close to the river where she died. After enough people are found murdered by the river, the villagers stop going there to get water or fish or whatever other resources the river provides.

The PC's roll around to the village, and find the villagers desperate for anyone to get rid of the river ghost. Eventually, they piece together the origins of the ghost. Now, the party can simply fight the ghost and destroy her, but the ghost is strong and dangerous. Alternately, the party can attempt to assuage the ghost's rage by getting Bill to apologize to her and beg her forgiveness - the ghost may either forgive/spare him or kill him, but either way, that will likely cause the ghost to leave.

On Bill's part, he would be extremely reluctant to go possibly sacrifice his life for the good of the village. But maybe he isn't as loathesome a person as he used to be. Maybe he really turned his life around since he's committed the murder and has become an upstanding member of society. He argues that the PCs should give him another chance, if at least to pay society back for his crime.

Thus, the PCs' role in the quest more than just casting judgment on one morally grey NPC. Symbolically, they are determining whether a crime can be paid for by good deeds, or blood must pay for blood.