My ex boyfriend is really starting to scare me?

ok so me and my ex broke up in febuary because he wasnt treating me right at all and before i broke up with him he said that if i ever broke up with him he would kill himself so that ade me really uneasy for a ew days and when i told my best friend what he said to me she got really mad at him and then the next day i broke up with him.... now ever sincei broke up with him he always looks at e like im crazy, he never takes things as jokes anymore, he gets mad way to easy, and just on friday my other friend hit him not even that hard and he just starting looking at her like he wanted to punch her he had his fists clenched and everything and he was slowly starting to walk towards her so i jumped infront of her and told him to back of so he just stayed in one spot nd staired at me luckly my boyfriend was there my boyfriend told him to back of so my ex just looked at him for a second and then walked off...... what should i do about my ex he's really starting to scare me... please help me