Because he is a psycho and sociopaths/psychopaths dont want to stop what they are doing, they thrive on control and cant stand to lose. I didnt manage to get rid of my xboyfriend for over a year, he constantly tried contact me and show up on my door, joked about how he would kill me, having thoughts out of the blue how he would kill me and another day laughing how he would dump me in water, and still he thinks nothing is wrong with how he used and abused me.

They know what they do but they dont care. You need to go NC and tell people who are your family and friends to not talk about him or give you any information about him. And if you should run into any of those who know him try ignore what they say because they will only feed you crap and then the bad cycles goes on. If you can try go places when and where you wont run into someone who knows him it will be good. They who knows him are either deluded or is in denial about who he is.