I think I can see where your anger came from since him with a baby in a picture probably triggered some old buried feelings of past bad times with your ex, and maybe a few more guys. But now its like you are trying to justify your react and that's not a good sign. It's a very unhealthy way to cope with the events you set in motion.
I can't blame you for looking at his Facebook, whatever your motives, but your hurt and shock were handled badly so acknowledge that part of your actions you could have controlled and don't shift the blame to him because you never looked for the truth after you found your evidence.
That's on you and no one else. Unpack that past baggage because it influences your thoughts and actions NOW, and the influence is not a good one. Deal with it, or you will repeat this behavior at the next "shocking" thing you encounter, and be mad and disappointed at yourself... yet again.