a fine mess

It is Clark's fault. I understand his concerns with telling him, wanting to keep it a secret, but that also meant they were not great friends to begin with I guess. Pete and Chloe found out about his secret on their own and that turned out well, even if it took Pete a bit longer to accept. But he was also upset that Clark never told him. Lex was more directly affected by it though, spending all that time and resources into looking what really happened the day of that car crash. I think that's why he reacted strongest to Clark not telling him.