What is wrong with my boyfriend?

His personality has completely changed in the last few weeks, i don't know what to do! Please help. He was normal when we first met but i noticed he was very stubborn and childlike and he couldn't show affection, now he is a monster, he abuses me every day calls me fat and says that I'm ugly, takes away my phone, slaps me, doesn't let me talk to friends and threatens me that he will kill me if i "talk bad about him". He is so nice around people who don't know what he is really like, but his sly smirk gives him right away. He can't apologize for anything or show warmth, he calls it weakness. He gets really furious when people sleight him in any way, his eyes look evil, i had to lock myself in a room once after i told him he was nothing more than a low life. He leaches off me for money and doesn't work, if i don't give him money he threatens and shames me. I know he had a bad childhood but he would never sit down and work out his problems, he seems to think that everyone thinks like he does and they can't help him without getting anything in return. I don't know what to do, I've never seen anyone so cut off from the world in my life, he is completely self absorbed and evil, what should i do?