I can't stand the thought of Snape being evil either, two many fantasies built around Alan Rickman as him, so. . . .
The letter said the TRUE Horcrux, could mean the others are plants, or more generally evilness/emotions or just will (like in the Neverending story) then peices of the soul. I'm with whoever said it was placed In dumbledore - what is more 'Hogwarts' than Dumbledore and what better way to control the place than by having a peice of soul in him? Dumbledore is all houses and all wizards - he is also the person that showed Riddle who he really was and what he was capable of, and with him being buried at Hogwarts - makes that peice of the soul still available and influential. I think Dumby figured this out as the soul started to assert itself more - As I was reading it I was thinking how Dumby was acting like the Riddle he was describing - he was becoming a 'collector' and looking to collect that which was most important to Riddle, including memories of his past - memories taken by Dunby just before people died, so he was the only one in possession. He even got the Slyth ring (despite a curse being used on him - forgot where it was mentioned, but Dumby did say the hand was the result of a curse, that had Snape not fixed would have gone further). Dumby realized what was happening and knew the only way out of his power being fully corrupted was death - As Dumby is also a host to Riddle, Snape is as beholden to him as he is to Riddle and so wouldn't betray him. Dumby begged Snape to kill him, to end it. Snape obeyed. (And I say, that in the time with Dumby, Snape has become reformed, realized what he did - and while still haunted by memories, has discovered the truth that love is more powerful - but is doing what needs to be done for the final battle to end. Remember, the Phoenix is a metaphor for all life - from the ashes of this major battle beauty will grow, it's the way it is in our lives, out of every war comes medical and technilogical advances that shepherd us into a new age, it is a fact of life and growth. Humans cannot live w/out conflict, it is as necessarry as food and water and air).
So I was worried going into the book, hearing the rumors. I knew the book had to be Snapes, I mean really - an old potions book found in Snapes old room? I actually liked it better than the last one.