My Step Dad Is Maltreating My Mom, I Am Confused

 

My step dad is maltreating my mom.  What should I do?

He comes back home very late and drunk and beats my mom almost everyday. I have told my mom to leave him but she is adamant, I even rented a house for her at a point so that she can leave him but my mom only spent about a week in the house before going back to my step father's house after my step dad called her and was crying on the phone that he will change.

Yesterday I went to meet her and I saw her face swollen. When I asked her what happened she dismissed it saying it's nothing, I knew my step dad has done it again. I have reported the case to the police several times but all to no avail. I don't know if my mom and my step father made a covenant reason why they can't leave themselves.

Why are men like this?  I am confused and heart broken