Guu wins.
Otherwise, Masao [Mars (manga)] and Akito Souma [Fruits Basket] deserve dishonourable mentions.
There's a scan about half way down
here with Masao grinning, but it's not really a good show of the soullessness and viciousness that he is capable of displaying sometimes. He's
not a nice chap.
Here's Akito:
It's mainly his voice, rather than his smile, that makes me shiver, though. That man is one of the most cruel and heartless people I know of in anime, and I love his Japanese VA's work on that.
And, for different reasons, Soujirou:
So little emotion invested in whatever it is he's doing or whoever he's killing.