Why did Snape take the UV in the first place?

[allies426]

No: HPFGUIDX 134505

I haven't seen this discussed extensively yet (although I admit I 
have not read all 1,000+ posts per day...).  This is my big question:

**Why did Snape take the unbreakable vow for Narcissa in the first 
place?**  He didn't have to.  He could have said, "Don't be 
ridiculous, I won't risk my life for your precious son's" or 
something similarly nasty.  It doesn't matter that Bellatrix didn't 
believe his loyalty to Voldemort, because VOLDEMORT believes it, and 
regardless of which side Snape is really on, he's the only one who 
really counts.  No further proof was needed.  Two possibilities, I 
guess.

Is he still on DD's side?  Had they discussed this already, and come 
to the decision that yes, Snape should take the vow, if asked?  How 
would they know it was coming?  And then the question again, WHY 
should he take it?  What's the advantage, other than proving loyalty 
to Bellatrix, when it doesn't matter what she thinks anyway?

Is he on Voldemort's side, and he really did want to kill DD, so of 
course he would take the vow?  I would be really disappointed.  

Allie