Why Snape's worst memory?

[Tonks]

No: HPFGUIDX 137230

--- In HPforGrownups at yahoogroups.com, "greg_a126" <grega126 at a...> 
wrote:
It seems eminently reasonable to me that the other two memories he
chose to pull out are the 2 concerning the prophecy; the one where
he heard it, and the other where he told Voldemort about it.
(Snip)
So is this Snape's worst memory, not because of what we saw, but what
we were about to see? We know that Snape pulled Harry out of the
memory before it reached its conclusion. Is there something
additional that we're missing?

Tonks:

Yes I think there was more that we did not see and it was the so 
called prank. I think what is missing is the fact that James saved 
Snape and now Snape has a life debit to James, whom he hates.  If 
you put this together with hearing the prophecy and telling LV, then 
you have a whole.  You also have the reason, IMO, why Lily didn't 
have to die, as I said in this post: 
http://groups.yahoo.com/group/HPforGrownups/message/134872  

This is Snape's worst memory because he is duty bond to protect 
James and he was also, later, the person somewhat responsible for 
James' death.  His worst memory is the night that he became indebted 
to James. 

This situation has messed up Snape's life. For one thing he has to 
protect Harry, he doesn't really want to, but the life debit 
transferred to Harry somehow and now Snape is stuck. We don't really 
know if Snape *wants* to be on the side of DD and the Order, or if 
he *must* be to save his own soul. By that I mean if he doesn't 
protect Harry and he is responsible for James' death, this is very 
bad magic indeed and I am sure that the consequences are very 
severe.  

After HBP Snape is even more stuck, poor man, almost like a man 
without a country, he is damned either way and has no place to call 
home. If you think Harry is a marked man, Snape is even more so. 
Whatever he does now will not matter, he is doomed. And it all 
started the night that James saved his life. Indeed there is 
probably more than one thing that is worse than death. Snape 
probably wishes that it had ended that night of the full moon many, 
many years ago. 

Tonks_op