Why do exes spread hurtful lies after a breakup?

Sort of a continuation of my last question. I was catching up with one of my exes friends because we use to be friends. We all work in a mall so I saw him one day and just stopped to say hi. Someone had seen me speak to him and went and told my ex.

He was not very happy and is now making up lies that have actually really hurt me and I just want to understand why?

He is saying he’s banging one girl after another now (obviously total bullshit) , and that we broke up because he was sick of fucking me all the time. When I literally have messages proving otherwise and me and my friends all know this was not the reason. Also in an effort to get us to stop talking , my ex told this friend that he still looks at my phone (we work together a couple times a week - 0 contact), and that I am speaking to the guy’s ex girlfriend which I’m obviously not.

My friends think he is still hurting over our breakup and the fact that I moved on and see other guys and I guess he hears about it but the things he’s saying about me have really hurt me.
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**and to just be clear , he’s telling the friend I am speaking to all of this stuff obviously knowing it’ll get back to me.
Why do exes spread hurtful lies after a breakup?
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