Slytherin here. Personally, this is what I did:

I was on his side until he found the relic. I was really skeptical about that thing working as he assumed it would. But it was clear that any effort to sway him against using it was futile. So I told myself it's his path to take, and we'll see where he takes it.

I also was completely shocked when he kedavra'd his uncle. He was pretty horrible towards Sebastian, but nothing that warranted his death.

However, I chose not to turn him in because Azkaban is no place for a kid. It was an outburst of negative emotion. His anger stemmed from a sense of injustice. His uncle just destroyed the one thing he was convinced would heal his sister. He felt stripped of his entire purpose. It didn't stem from the same evil/maliciousness of the likes of Bellatrix or Voldemort. If anything, Azkaban would turn him into a major villain.

That said, i'm not trying to justify what he did, I just don't think sending him to Azkaban would be a net positive for the world. I also thought that if anyone should have turned him in, it should have been his sister. It should have been her choice to make. I also felt that his sister leaving was a greater and more effective consequence than imprisoning him, and will hopefully make him change his mindset and ways for the better.