@KRBlack
It's actually implied if i remember right that yu has no desire to torment people for the sake of it (at least not anymore), but still has her sadistic scientist side when dealing with people who go against kunai, though it's implied that she won't really go all out of the darker side of her nature unless they really piss her off, which, considering she's implied to be quite fond of aku and luna, not to mention how fond kunai is of them, probably has something to do with just how sadistic she is in this scene.
Basically it's likely she wouldn't have been nearly as cold blooded if he wasn't the child torturing monster he was.
She's not a "noble" character, but she does have her own set of morals and they pretty much align with her boss.
I think the implication is that they changed along with kunai's personality change, thus their morality comes out similar to his, though they still retain some of their traits, they are essentially separate to a extent from their originals.
@Serenata
Pretty much, yu had the potential to kill before but pointed out she didn't really enjoy blatant killing if i remember right, this guy on the other hand, well, let's be honest, half of us would want someone like this to die a slow, painful death for all the stuff he'd done.