after the first reading...

[hogwartsmom]

No: HPFGUIDX 133610

Mira <anurim at y...> wrote:

 Why did he [Snape] make it [the unbreakable vow]? I believe that
mostly out of pride. 

HogwartwMom says:

I think he was trapped in the situation with Bellatrix and thought he
could ameliorate the situation by vowing, then, when Narcissa added
the last part of the vow (a surprise to Snape, he hadn't bargained on
that), he couldn't back out. 

The situation at Hogwarts got out of hand because Draco didn't tell
Snape about the cabinet.  Had Snape known, he could have been in more
control of the situation.  As it was, he and DD's "worst case scenario
plan" had to be used.

HogwartsMom