Why, if I left my ex alone is he so harsh and mean to me? He left me?

I'm totally baffled by my situation as I can't understand it. Me and my ex had a good relationship over all. We were happy up until the last month or so when we kept arguing. He felt it was affecting his work and didn't want to work through it. He agreed he still loved me to some extent but obviously not enough to work through it. His words were it's not working. I made it clear it wasn't what I wanted but agreed it was draining. From then on I never contacted him. A week or so later he sends me some nasty message calling me grim for getting with somebody else. I ignored as I don't need to reply to something as nasty as that! I didn't get with anyone to start with. He had immediately started speaking to the girls he saw before me and they told him I was with other people, which I wasn't. A month later he messages me to tell me to stop talking about him and that he didn't speak about me so I should just leave it.. He also added I'd slept with four people and he knew I used to be an escort. None of this was even true and all I've done is leave him alone, not say a bad word about him and I haven't been with anyone else ether. I told him what I thought and blocked him. It's actually caused me anxiety as I never wanted him to think that way of me but I'm starting to feel he's not a great person and doesn't know me very well if he believes that especially because he's speaking to his ex who never has a nice word to say about me because she didn't like to see him happy with me and she's fairly toxic to everyone anyway. All id like is some thoughts on this situation? Please do not tell me to not contact him, as I won't and haven't. I just want to know from someone else's perspective why he's been so cruel and cold.. Gone out of his way to hurt me when he ended it and it's what he wanted?
Why, if I left my ex alone is he so harsh and mean to me? He left me?
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