Why do i do this?

Okay, I'm Casey. I'm a great kid, I have many, many friends. But I have one enemie that I need to face and try to understand. I'm seventeen years old, I live with my grandmother. I grew up from birth to the age of twelve with my biological mother. Before I could even walk I was being physically abused. I used to wake up to beatings at 2-3 in the morning. Some days were worse than others, beatings from sunrise to sunset. I was not only physically abused by mentally and verbally. As well as sexually. Mainly by Kim(my biological mother). See my mama is a hellish woman from pergatory. She is a pathological liar and just a terrible demon. I grew up with my sister Kayla. She's all I had, all I needed. I Love my sister. She's always been there for me, I never had to ask for her to be there for me. She just was. We were inseparable. See Kayla has been raped by many different men during our childhood together. I never knew about it. As we grew older things became more and more clear to me about who truly loves me. Kayla has the worst case of P.T.S.D. That children's hospital has ever seen. They used her and ran tests all the time to understand her condition better. Now, when Kayla "turns" part of her brain shuts down. She developed the mind of an infant with the body of a 18 year old. Kayla has many, many full body seizures. My father, was never around growing up. He was too busy living the rock star way of life. He was terrible on drugs and alchohol. So was Kim. She still is, but my father is clean now. In 2002 my father fell from a 37 foot building while on a construction site. He was a master carpenter. He landed on his head and it's a damn miracle I still have him today. There is much much more to my past but I'll get to my point now... I haven't been angry in years, well at least I haven't blacked out from anger in years. 3 years it's been. But when I black out and snap, I hurt the person I'm angry with. Never my family members or friends. But to those that hurt me hen I have not hurt them. After I hurt the person or persons that I'm angry with. I begin to cry. I don't sob like a little girl. I just shed many tears. It would be a miracle if someone could help me on my search to understand why I cry after I blackout. Please if you know anything, tell me. Before I go, my grandmother is my savior. She took me from being on the streets with no food, shelter, or love. And gave me it all. My grandmother is the closest woman to perfect I'll ever know. God bless you mom. I Love You.