Many victims of abuse feel they can't leave. Their parents controlled everything. They knew that their father was a powerful man who had the resources to track them down in if they left; they didn't think they could successfully escape their parents, nor did they think anyone would believe them. There are people who kill their abusers out of fear, and they often do so when the abuser is vulnerable because the victim knows that they can't defend themselves during a confrontation, but that does not mean that the abuser has not been or is not a threat to them.
The brothers stated that they believed that their parents were going to kill them that night. They realized after the fact that they were mistaken but they still believe that it would have happened at some point. Jose was not going to let his sons live to expose the abuse or to leave, because he would be ruined and he would lose control over them. Kitty was Jose's partner in abusing their children and she too, would not have wanted the truth to come out. Lyle stated that he believed that their parents would kill him and Erik, and then tell people that they ran away - which is not far-fetched when you consider domestic homicides that have been covered up.
Manslaughter would have been an appropriate conviction in this case - 25 years - 10 years for each parent, plus a weapon charge - which means that the Menendez brothers were not going to "get off". That's the misconception that many people have about this case - neither the brothers nor their attorneys ever claimed that the killings were justified, or that the parents deserved to die. They were just trying to explain why it happened.