I have a fair number of headscratchers about this game..

[hollowleviathan]

Wasn't the kid's death is the last drop and a reason for Amnesia's potion?

There's never been any hint of what else the Amnesiac potion is made up of aside from Damascus Rose, and blood has never been mentioned for that either, as far as I can remember. The main importance of blood was just for vitae in this game.

I'm pretty sure they just meant the last straw, the final depravity that snapped Daniel's mind and made him realize he could not continue to participate in this, and needed to kill Alexander.