Snapedom

Jodel has a very nice analysis about what was going on behind the scenes in PS:
http://www.redhen-publications.com/QuirrellDebacle.html

I agree with a lot of it, but as a Snape fan I have a problem.

Quirrell was first an agent of Riddle’s and then possessed by him. He was working in Riddle’s interests, and trying to steal the stone.

And trying to kill Harry, on the side.

Now, in retrospect it’s clear why that fact wouldn’t much worry Dumbles. But Snape? The man who’s pledged his life to “help [Dumbles] protect Lily’s son”? Why did Snape continue to go along with Dumble’s insistence on setting a clever little trap and spending months trying to lure Riddle into it rather than getting rid of the man, after the first time Quirrell tried to murder Harry?

When did Snape realize Quirrell was Tom’s agent? When did he realize Quirrell was Tom’s possession? And why was he content to do no more than just “keep an eye on Quirrell” and interfere with such murder attempts as he spotted? I mean, keeping his cover only goes so far: what's Snape care if his reputation with Tom is intact when Lily's child is dead?

What did Snape think was going on?

Answers, anyone?