Why did Daniel drink the Amnesia potion?

[superluser]

Simple.

Daniel is outraged that he has been manipulated into doing things that he now regrets. Were he to kill Alexander now, it would be an act of rage against his partner in crime who had duped him.

But if he forgets, he will be carrying out an act of righteous vengeance against a dangerous and violent man.

He may also not trust himself to do the right thing in regard to the orb. His amnesiac version of himself would presumably be less prone to that desire. Daniel had seen the profit and become inured to the evil. His amnesiac version would see the evil, which would insulate him against the profit.

The irony is that Daniel is duping himself into committing acts for purposes that he does not fully comprehend, which is why he decides to take action against Alexander in the first place. Or perhaps he rationalizes duping himself by the fact that his hated version of himself will be lost forever.