Why did Hugh’s brother kill himself after killing their parents?

SPOILER!
____________________________________________________________________________________________________________________________________________________________________________________________________________________
I think the novel took a different way explaining it, but here is the gist of it:
The official story goes: Our Hugo was selected as the heir by his father, his brother abandoned. Later his brother reurned and killed late duke and duchess while our Hugo was away.
Reality: Our Hugo was the one abandoned, but later met his twin brother. His brother was the only important person to our Hugo. The Father used the brother to pressure our Hugo into becoming the heir bc the father thought that our Hugo was more fit for the position than his kind brother. The brother then killed the duke, duchess and himself, thus freeing our Hugo from that messed-up family.