Hello. I read in the walkthrough this quest involves several steps like meeting Gareth, investigating his parents ' murder , fighting or convincing some authorities, etc...
Problem is, without taking the quest I directly find the broken house, with several enemy corpses in the doorstep and Gareth about to execute his former friend.
Is It a bug? Also, Should I convince Gareth to spare him or to kill him?

There's a second house later. That's just the first step in the quest, you're fine. As for which way you want to go, that's up to you to decide. But your decision may have consequences later.

Kill him. That bitch gonna appear later on and fight you if you let him live.

A good Magister is a dead Magister.