What if Snape and Reg decided to leave the DE's at the same time and did some sort of Unbreakable Vow about it?

Ooh, I like this idea!

I have always imagined that Snape was envious of Sirius, because Sirius had the family position and wealth that Snape coveted, but Sirius didn't want it. I imagine that Regulus and Snape were friends. bethbethbeth wrote a lovely fic about them.