“Wonder if they are entitled to any benefits from their parents will.”
Typically, no.
There is typically something called the “Slayer Statute” that treats a murderer as having predeceased the victim.
That said, I believe the money went to the immediate family, who then promptly put it into a trust for the brothers, as they considered the murders justified, which tells you something about how crappy the parents were.
The parents weren’t the problem. They murdered their parents, collected the life insurance and went on a wild spending spree till they were caught.