Why JKR had Dumbledore trust Snape.

[Run Away!!!]

Not for certain, but whether he is good or not there is still something nasty about him.

[aquamonet]

Everytime I read about this point I'm split two ways.

Either, Snape killing Dumbledore was something that he had to do to not blow his cover and Dumbledore understood this. May be there is something about Harry not being able to truely face Voldemort while he believes that Dumbledore is going to be there to save him. In this case while not being a nice person Snape is still on the side of good and therefore Dumbledore was right to trust him. For this to work I'm sure Dumbledore knows something about Snape which we don't. It would fit with other events where we've thought Snape is the bad guy and he wasn't (Quirrel and the Quidditch match etc).

The other idea is that Snape is evil in some way. I don't think he's as loyal to Voldemort as we think. May be while Harry Potter is alive he feels that he holds some power with Voldemort but once he's dead he feel he'll get left behind. In this situation he must therefore have a plan. Possibly he holds the last horcrux or something along those lines. Here the betrayal of Dumbledores trust has been used to show us that even heros make mistakes and not everything is always going to go right and there is no such thing as an obvious good and bad. This works with other part of the Harry Potter books where bad characters haven't necessairily served under Voldemort but have been evil for their own agenda.

Again, this is just speculation and I'm sure JK will come up with something different again.

[GodrictheGriffon]

He might even still be deciding what to do. Is DD the man who gave him a second chance or the man who kept him from his true passion, DADA teacher, for such a long while.