That "Wonderful" was a ploy to get into his pants, which I'm pretty sure she did.

Seriously though, I agree with Buff. It does seem kind of OOC for McCunnen to praise Zwei for following orders blindly. I'm pretty sure Zwei did not wish to kill the child, and would have avoided it if he could. But he still did it because he was ordered to do so, much like Ein.

The only reason I can think of is that McCunnen personally thinks that Zwei finally decided to do anything to further his rank in the organization, and has willingly accepted his fate in Inferno (which I very much doubt is the case), and praises him for it. She did say that even if the path was determined, Zwei gets to decide on the speed. Killing children kind of speeds up the process of becoming a cold-blooded assassin.