Snape's and Black's grudges (was: Living with Sirius)

[Cindy C.]

No: HPFGUIDX 28326

Gwen wrote:
 
> Now, we come to the night Black takes action, kidnaps Ron, and 
lures Harry
> to the Shrieking Shack, presumably to witness Pettigrew's capture. 
Lupin
> joins them. Snape, acting responsibly, goes to find Lupin, sees the 
tunnel
> in use. And what does he do? He also doesn't summon Dumbledore. He
> immediately goes, he thinks, to aid the kids.
> 

I had a different take on what Snape is thinking when he goes to the 
Shrieking Shack.  After he takes off the Invisibility Cloak, Snape 
says:

"I've just been to your office, Lupin.  You forgot to take your 
potion tonight, so I took a gobletful along.  And very lucky I 
did . . . lucky for me, I mean.  Lying on your desk was a certain 
map.  One glance at it told me all I needed to know.  I saw you 
running along this passageway and out of sight."

Now that I look at Snape's statement again, it seems a little 
strange, doesn't it?  Snape's statement that Lupin went out of sight 
suggests the Map doesn't display the path all the way to the 
Shrieking Shack.  So Snape can't know that Black, Pettigrew, or HRH 
are in the Shrieking Shack until Snape arrives there himself.  (Snape 
saw Lupin go out of sight, so everyone else was already off the Map 
by the time Snape saw it.)  Perhaps we should rule out the 
possibility that Snape goes to the Shrieking Shack to assist HRH or 
to capture Black, because he doesn't have any idea they are there.  

But what does Snape know for a fact when he leaves Lupin's office and 
heads off for the Whomping Willow?  Snape knows that Lupin is headed 
toward the Shrieking Shack, headed toward the place Snape knows Lupin 
used to use for his transformations.  Snape also knows Lupin failed 
to take his potion.  Snape also knows (probably) that a full moon is 
due.  It would be reasonable for Snape to conclude that Lupin is 
headed for the Shrieking Shack because Lupin suddenly realized he 
isn't safe, and wishes to avoid transforming in the castle and 
endangering others.

So what is going on here, and why does Snape follow Lupin?  Is Snape 
behaving the same way he did years ago, snooping around after Lupin, 
hoping to catch him doing something untrustworthy?  Well, Snape did 
take the Invisibility Cloak with him, so he certainly had the 
intention of "snooping" or observing Lupin while Lupin doesn't know 
Snape is watching.  Although I had previously given Snape credit for 
rushing to the Shrieking Shack in his role as watchdog over the trio, 
I am now starting to think Snape's actions are just another attempt 
to harrass Lupin again.

Does anyone have a theory about what exactly Snape is thinking here?

Cindy (who is still holding a mini-grudge that Snape criticized 
Lupin's teaching methods in front of the students)