Regarding the Menendez brothers (a case I have studied), one thing I will say is that their father was a monster. Their mother was an emotionally unstable, and dependent personality who refused to divorce her philandering, controlling husband.
Of course, it doesn't warrant murder, but I believe this case is more complex than it appears on the surface.
For starters, you have to look at Jose's background. He was the only son; his mother spoiled him tremendously and refused to discipline him. From an early age he was a bully, biting his playmates when he was younger and punching them when he was older. He grew up believing that he was better than everyone else and carried this into adulthood. While he worked hard to rise up in the ranks, he was very much disliked by many for his ruthless drive for success and he expected his sons to be just like him. Mary Louise, known as Kitty, Jose's wife and mother of their two sons, frequently witnessed her father beating her mother as a child and after her parents' divorce her mother became a withdrawn alcoholic who became dependent on her children to take care of her. She was distant and moody, and Kitty, the youngest child, particularly felt neglected and unloved. Unfortunately, both of them repeated these cycles with Lyle and Erik.
Jose didn't view his sons as individuals, but as extensions of himself. They were not valued as human beings. Perfection was expected, both in athletics and in school. In fact, Jose taught his sons that winning and being the best was most important. Do anything, even lie, cheat or steal - just don't get caught. Kitty often did their homework for them, and Jose would intimidate their teachers into giving them higher grades. That they might have had learning disabilities was a possibility Jose refused to consider. The boys' teachers and coaches called them "the parents from hell". Even when the boys were small, Kitty was very neglectful - she'd have them wearing dirty diapers all day long and later would leave them unattended for hours at a country club in New Jersey. Lyle and Erik were very immature for their age, something that their teachers noted. They both showed signs of disturbance and behavioral issues; coaches and family members witnessed Jose hitting his sons and berating them for not winning a tennis match. Lyle wet the bed until he was 14; he slept surrounded by stuffed animals, even as a teenager. Erik was so fearful as a child that he frequently checked his closet, under his bed etc., for "monsters". Both of them had stuttering problems; Erik's was so severe that he went to a speech therapist. Kitty said more than once, in the presence of others, that "children can drive a wedge between their parents". Her friends noticed her jealousy of her sons, especially Lyle, for taking her husband away from her. More disturbing are photos taken of the boys when they were in elementary school; in the shower, and in most photos, the focus was on the lower half of their bodies.
Here is a photo (I apologize that it isn't better quality) that is often shown in documentaries and books about the case, but is almost always cropped so that we don't see that Jose has his hand on Lyle's crotch:
We'll never know whether they were sexually abused or not or if it was a real factor in the killings. I suspect that greed was a part of it, but not all. What can be said is that the Menendez family was NOT normal by any means and the brothers never lived in reality, but rather a world that they were born into. Sexual abuse, particularly incest, is often referred to as "psychological murder", and it's clear that they were disadvantaged at an early age, despite the wealth and privilege they experienced. No real world experience, just the unreal world where violence was tolerated and winning was everything. That they came to resent that is not surprising, but of course, they had a choice in what they did, now they must live with the consequences.
