divorce under what grounds exactly?

My husband and I got married 07/28/2012. I already had a daughter (my husband is not her biological father) and I was pregnant for our son together. In July 2014 my husband punched me in my face, hard enough to leave it swollen and bruised and family witnessed the swelling and bruising. In September 2014 he moved me and the kids all the way across the country away from all family and friends. (Moved from Northern NY State to Washington State). In October 2014 I found out and he openly admitted to having sex with my 17year old cousin in June 2014. (So he cheated on me.) Just 2 days ago I found out I am pregnant again. But I feel the arguing is unhealthy for me and this pregnancy. I want a divorce and have wanted one since October. He calls me names and tells me that him cheating on me was my fault because of my weight and that I do not give in to him every single time he wants sex. He degrades me. I tell him i want a divorce, and to leave. He threatens to not let me take our son with me and says that he will make anything up that he has to so that the courts will grant him custody of our son over me. His family also harasses me. They call me names, call my daughter names ("bastard-child"), they threaten me as well. My husband has not once stuck up for me or my 3yr old daughter against their harassment and gets angry at me when I ask him to. He blames their harassment on me saying that I have "a bad attitude" and then changes the subject. I have been nothing but nice to them people. I want a divorce from this man and his harassing family. Under what grounds am I legally able to file? And he says he won't let me take my son if I leave. What can I do about him threatening not to let me take my son and saying he will lie to courts about my mothering?