Disclaimer: Harry Potter and all characters therein belong to J K Rowling, not me.
What He Wanted
Rodolphus is the eldest son of the family. He always gets what he wants.
Rabastan is used to it. He's used to coming second. Rodolphus claims what he wants, and Rabastan stands back and lets him; the spotlight, the notoriety, the striking daughter of the House of Black.
Just now, watching from behind his mask as Rodolphus tries to keep Bellatrix from charging into yet another fight too soon without provoking her into flying off the handle at him, hearing her bloodthirsty shrieking, Rabastan doesn't mind at all.
Rodolphus got what he wanted, and it was exactly what he deserved.