Why did Snape take the UV?

[justcarol67]

No: HPFGUIDX 156866

 
> > Nikkalmati: 
>  We don't know enough about the UV  to know if SS had any choice but
to swear to part 3 once he had begun.  
> 
> 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.

Carol, who thinks that both Snape and DD knew about Draco's "job" (but
not about the Vanishing Cabinets) and that Snape told DD about all
three provisions of the Vow