Why do you think my grandmother stayed in her marriage?

My grandfather is a very mean spirited person, and was a bad alcoholic until he reached about fifty years old. My grandmother tells me she stayed because she didn't have a mother to help her and her father also died at a young age. Anywho my grandfather kicked out my uncle when he was a teenager and my uncle's body was found dead two weeks later. The day after his body was found my other uncle broke his neck in a diving accident and my grandmother had to spend the next 13 years caring for him full time before he died. Anyhow, it just boggles my mind that they're still together, they are both so mean to each other, and she blames him for both of their sons' deaths. Does this seem crazy to you? Or was the stigma of divorce that bad? She confessed to me on several occasions that she thought about killing him, and even had a plan, she was going to push him down the stairs one night when he was drunk. She says she didn't do it because one of her children was so young at that time.