Most Helpful Guy
It means he's immature and trying to scare you. He probably will badmouth you for a while and may come back to bother you. But basically all it indicates is that you are right in breaking up with him
Most Helpful Girl
I would tell your brothers, uncles, dad, but friends, so they can show him you aren't alone. If he knows where you live, either make a police report at your local police station (because that's a threat) or hire private security to watch your house for a bit or hire someone to scare your boyfriend.
Also I would take self defense classes in case he wants to harm you/ kidnap you.
If he knows your daily routine activities, I would change it up. So he doesn't know where you are at a times.
Change your phone number.
A way I get even with people is I post their number on the w4m personal section on craigslist. Posing as a girl wanting dick pics. The phone number usually gets flooded with fuck pics and unwanted phone calls, that the person has no choice but to get a new number. If you do this though be careful which email you use, and use a fake phone number for the confirmation code. Leave no evidence of anything that can be traced back to you. Greatest way to fuck somebody you don't like over. The app I use for fake phone numbers is textfree. Good luck anyway if you decide not to fight back