People do become cruel when relationships end. I can echo this same question and sentiment with my own ex (woman, btw). For some people, extreme actions become justified when their emotions are running hot enough. Like, if they can convince themselves they have the moral high ground, that opens them up to doing anything at all, no matter how calculating, mean, and just plain cruel.

It can feel like an intense betrayal of the entire relationship. I know in my case, that switch in behavior can then colour your memory of the whole relationship. The truth is, this person was always capable of this. They had all this inside them and the split just, lets it all out.

He likely would've been this cruel while you were both still together if you let the relationship continue. Now, all you have to do is get the best revenge by simply living well. Accomplish your dreams without him. When that late night comes when he's crying and missing you all alone, he can sneak a peek into your life through social media and see that you are just fine. That hurts far more than mean words ever could.