Why Snape's worst memory?

[greg_a126]

No: HPFGUIDX 137209

After HBP, I have to go back to wondering *why* the memory we saw in 
OOtP was Snape's worst.  We know that he stuck 3 memories into the 
Pensieve.  He was obviously afraid of Harry doing what he did on at 
least one occasion and get into his head (where he saw the memory 
Snape had of himself watching his parents fight).  

It seems reasonable to me to assume that he had a number of memories 
in his head that he would prefer Harry not see (such as the one Harry 
did end up seeing) but to pull every unpleasant memory out of his 
head and then stick them back in there would take so much time that 
it just becomes impractical.  Therefore, the memories he specifically 
chose to pull out must have some extremely important relevance.

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.  It 
stands to reason that eventually Harry was going to have to hear the 
prophecy (it is about him after all) and that when he did, he 
wouldn't appreciate Snape's role in his parents death.  

So since there are 2 memories he's pulling out b/c of self 
preservation, that must mean that the one we saw in OOTP is worse 
than everything else he's ever done in his entire life.  No bit of 
killing muggles, or anything else awful that he did while a Death 
Eater, no amount of abuse from his parents, not the time he was 
nearly killed by a werewolf, not any other time that Sirius and James 
picked on him, this one time was apparently infinately worse.  

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?

greg