SureAI

The Mayor's Brother was innocent of the murder, and they were already avenged by killing the mercenary that disobeyed him and killed them. His death was unnecessary and unfair.

it depends, if this was the current day and in the U.S. & Meldor was telling the whole truth, it would still possibly be negligent homicide at least (setting in motion events that could lead to homicide in a foreseeable manner), if not felony murder depending on how the original crime is counted, mainly the intimidation. if the charges were dropped down, still there's accessory to murder, conspiracy, assault, and manslaughter. that's hardly innocent.

---edit
damit, now i sound like a blood sucking vampire...

Yes, they were avenged but justice wasn't done. If I recall correctly you don't sentence him to death, you merely tell him that he needs to turn himself in and go before a court. He doesn't though, he tries to kill you because he doesn't want anyone else to know so the woman kills him.

This quest is quite interesting to me, I mean why is this woman so unique. She wasn't an arcanist or a wild mage. Why did she, out of all the rest of the people that suffered great injustice raise from the dead to torment the guilty party.

I didn't question that at all, but then maybe I read and played too much of the Witcher. A murder woman raised as a ghost avenges her and her lover's death by gruesomely killing the one who caused it all? Sounds so normal to me.

Maybe the woman actually had magical abilities and nobody ever found out?

What I mean by that is she's unique, in the whole game there are no other instances in which a similar thing happens. Unless you count the Lost Ones but aren't they connected to the Cleansing?