Ex from 3 years ago suddenly sends an enraged message to me

I am 2 weeks postpartum and somehow this guy found out I have a baby now and decided to send a hateful ragey message to me. I happened to be up feeding my infant. Like….. what? What is wrong with this person? I genuinely feel very nervous and uncomfortable.

For context: we dated for less than a year, I broke it off well before pursuing my current relationship. He did continue to call/text/send me mail/letters after I broke it off which was stressful so I can’t say it’s a total surprise he did this but it just seems so unhinged.

Some quotes from his rant:
“So glad you found happiness in someone else after you said you loved me”
“I’ve never hated someone as much as I’ve hated you and I wish I never met you”
“You lost so bad and I’m so jealous anyway”
“If you ever miss me go fuck yourself”
“I hope you are living the life you’re accustomed to”
“You’re used goods”
“You’re such an awful person”
“Remember how we talked about kids and now you have a kid with someone else?”

He also said I “cheated” even though I didn’t.

For the record, I didn’t start seeing my current partner whom I now have a child with, until 4 months after I ended things with that guy because he was being an absolute nutcase during our relationship and I couldn’t take it anymore. It took so long to break up with my ex because he wouldn’t stop calling and texting and refusing to accept it was over.