My fiancé just slapped me...

My fiancé just slapped me. He's drunk and we were driving back home from his families Halloween party. I was upset because I caught him doing drugs in the past (cocaine and weed). Before we left the party he was talking with 2 other guys that sell and smoke weed and do cocaine he told me to wait on his families porch while he got the guys "cellphone" that he left charging in his truck and to me to wait. Well me being hard headed and not stupid I went there anyway and it looked like my fiancé bought something and hid it in his truck really quick when I walked up. No fucks given. The guys left and I told my fiancé let's go home now. Without saying goodbye. I argued with his suspecting he was back to his old ways and he slapped me. He has never laid a hand on me before. I don't know what to do. I screamed at him when he did it and he threatened to slap me again! WTF!!!!! I want to leave him but I don't know. It's hard. Someone please help me leave him.