Because he can, I think. He's working for Aogiri Tree, but he does it for his own scientific intrigue.

[Takizawa would never pretend for a moment that Kanou was a good person. Not in any way. He'd put him through too much.

He turns his eyes down to his hands, thinking of the treat he'd eaten just minutes ago. Has Kaneki been having the same thoughts as him? He'd claimed he'd eaten already.]

...What do you smell, Kaneki-kun, when you smell the food here?