Why did Snape take the UV?

[a_svirn]

No: HPFGUIDX 156867

> > a_svirn:
> > Well, Arthur Weasly was able to stop Fred and Ron after they had 
> > already started the ritual. 
> 
> Carol responds:
> Stopping two five year-olds who don't really know what they're 
doing
> is completely different from stopping two Dark witches like Bella 
and
> Narcissa. Also, the kids probably hadn't reached the stage where 
Ron's
> hand was bound to George's with rings of fire. (Personally, I think
> Arthur heard them trying to make Ron swear to do something and saw 
him
> Ron and George on their knees with Fred standing over them with a 
wand
> but stopped them before any harm was done.) Even if the kids had
> reached that stage (and surely even Fred and George weren't that
> talented at Dark magic at age five!), Arthur wasn't one of the 
parties
> to the vow. He was an armed outsider (meaning he had a wand and was
> not a party to the IMO uncompleted vow) and could apparently undo 
the
> damage, if any. 
> 
> But Snape is in Ron's position, that is, he's the person making the
> vow, and the Bonder is not a five-year-old Fred but a loyal DE who
> suspects him of disloyalty to the Dark Lord. He's kneeling on the
> floor with his wand hand bound to Narcissa's by a double ring of 
fire.
> Even if he could grab his wand left-handed and somehow undo the vow
> (which I don't think is possible since it's Unbreakable), it would 
be
> fatally stupid to attempt it with Bella standing over him with a 
wand,
> not to mention that it would prove to both women that he was in 
league
> with Dumbledore and they would find some way to report him without
> incriminating themselves.
> 
> I think that once he's taken the first two provisions, he has no
> choice but to take the unanticipated third one and hope there's a
> loophole.
> 

a_svirn:
But if you are right, then taking an UW, *any* UW, is unmanageable, 
and by definition unpredictable risk. Let's see: once you start you 
can be made to swear just about anything. As soon as you are on your 
knees and the ritual is started you are at the complete mercy of the 
persons involved. They could change wording without warning, come up 
with provisions at will and so on. How you can even begin to assess 
risks with such an arrangement? Of course, if you trust second and 
third parties implicitly it is another matter entirely. But we don't 
have to consider this eventuality. Not at the Spinner's End, at any 
rate.