Snape? (HBP SPOILER)

[chrusotoxos]

No: HPFGUIDX 141311

Hi everyone, and sorry about this message, I'm sure you've 
discussed the matter over and over again, but I've been out of 
touch for a while and have a lousy connection. If you know of a 
message already answering this, I'll be glad to read it.

Ok, so my problem is (how can it not be?) whether Snape is bad 
or good.
My theory (and everybody else's, I gather) is that he killed 
Dumbledore on Dumbledore's orders. Possibly DD was already 
dying , weakened by the ring's curse, or simply by old age. And 
he thought it was more important for Snape to survive 
(otherwise, he would have broken the Unbreakable Vow) than 
himself, which means Snape has a great role to play now.  But if 
so, DD was unwise not to tell Harry a better reason for trusting 
Snape, because now Harry wants to kill him. Also, nobody else 
knows this reason, apparently.
And Snape must have hated DD for forcing him to kill him, the 
only person who ever believed he was worth something. All that 
shouting, Don't Call me a Coward in the end points in that 
direction. If a bad DE, why Snape would have stopped his flight 
to answer Harry?
But there is a big hint against this, when DD says "I make 
mistakes, and being smartier than other, my mistakes are much 
bigger"

So, what do you think out there? Snape is DD's mistake or not?

See you
chrusotoxos