I reckon his current toon already reflects that. I mean, look at his abilities. His basic can't be avoided by Jedi, because his whole thing is hunting them down and showing them no mercy. Force Crush has gotta hurt, and the three stacks of DoT seems to me to represent internal injuries. That's pretty dark. And then you have Culling Blade, which finishes off enemies that are already injured that way.
The Vader we have now is already cruel and dark. He definitely isn't the Vader that held back when fighting his son.