Ex boyfriend

So my ex and I broke up officially in February and since then he's put me on an emotional roller coast. I don't have feelings for him what soever but he's finding a way to keep himself relevant in my life. I tried doing the whole let's be friends thing but that blowing up in my face. He's really upset that we can't be together and I told him that it is what it is and I don't want a relationship with him but he's not taking no for an answer and he's really scaring me now because he's been talking about if we don't get back together he's gonna kill himself and even tho I don't love him like that anymore I don't want to see him hurt himself because a little part of me still cares because I don't want to have that guilt on my conscious. He told me last night that he doesn't want to see me have sex with anyone else but him or he's gonna kill me and I took that offensively because even if he was joking people are crazy now a days and I'm not trying to be the girl murdered by her ex on the news. I'm trying to tell him that there's someone better but he doesn't want to listen and I honestly don't know what to say. PLEASE HELP ME IDK WHAT TO SAY TO HIM OR TELL HIM TO LEAVE ME ALONE. Because he blows up my phone and texts me and my mom and it's just honestly too much. I need advice.