He killed 3 people from what we’ve seen.
Well, I don't think Alex committed those murders. There were too many inconsistencies in his stories. I think the true murderer is Clark, and Alex, fueled by maybe the trauma from possibly killing his father, has spun elaborate fantasies because he actually thinks he is the killer.
With that being said, I think that this is on a subconscious level for Alex and maybe he doesn't realize it. But because he thinks he's a killer, I think that he has begun to take on the traits of a killer.
Why is it Clark, tho?
I'm pretty sure the only murder Alex ever committed was killing his father.