[Orofacion of the Vanyar]

Sauron used her to guard the pass...

This isn't quite fair to say. Sauron had little control over Shelob, her presence in the pass was just a bonus that profited both, a sort of symbiosis.

I find Shelob an interesting subject. Since Ungoliant was a Maiar corrupt, and the spiders here spawn, this means they are children of maiar. The only other child of a maiar was Luthien. An interesting parallel to say the least.