[Gently:] You took Winterfell, betraying king Robb Stark. You killed- or had someone kill two farm boys in place of Bran and Rickon Stark. Your ironborn men turned on you rather than stay loyal to you- then you remember no more... that much is shared?

[It's not quite as good as sticking a knife in the man, but it'll do for now.

The key was to get Reek to agree with how Ramsay was defining him. There was power in that- a dog would never properly obey until it understood that it's master was leader of the pack and that the dog was to be obedient under all circumstances.]