How we cite our quotes: All quotations are from Harry Potter and the Half-Blood Prince.
Quote #4
HARRY: Did you know, sir? Then?
DUMBLEDORE: Did I know I'd just met the most dangerous Dark wizard of all time? No.
Dumbledore clearly could tell that Voldemort was trouble—after all, he immediately called the kid out on stealing from a classmate. However, he'd obviously been hoping that Hogwarts would help shape the kid into someone…slightly less, you know, sociopathic. No such luck, though, we're afraid.
Quote #5
HERMIONE: Do you think he would have done it? Draco.
HARRY: No. No, he was lowering his wand.
The big revelation of this particular film: Draco Malfoy isn't pure evil. Mind. Blown. We would have bet on the opposite in a heartbeat up to this point, but nope, he's ultimately unable to kill Dumbledore, which suggests that he's not all bad.
Quote #6
DUMBLEDORE: Draco, you are no assassin.
DRACO: How do you know what I am? I've done things that would shock you.
DUMBLEDORE: Oh, like cursing Katie Bell and hoping that in return she'd bear a cursed necklace to me? Like replacing a bottle of mead with one laced with poison? Forgive me, Draco, I cannot help feeling these actions are so weak that your heart can't really have been in them.
DRACO: He trusts me. I was chosen.
[…]
DUMBLEDORE: Draco, years ago, I knew a boy who made all the wrong choices. Please let me help you.
DRACO: I don't want your help. Don't you understand? I have to do this. I have to kill you. Or he's going to kill me.
Dumbledore is calmly but desperately trying to keep Draco from heading over to the dark side, and Draco is desperately trying to convince himself that he wants to go there. As you can see, Dumbledore doesn't think Draco is all that into evildoing, and if he commits murder…yeah, it's going to be hard to come back from that.