Well I have only played past leaving Hinterlands and done a little main story after that, but I play with at least 3 different characters (different class for each). And so far (and after reading a bit more on the forums on the opportunities), very little compared to how ruthless you could be in DA2 or Mass Effect 2 and 3.
It seems to be comparable to DA:O and ME1. I guess it's because they switched from using quite a lot of cutscenes to using few, because most dialogues you have are non-cutscenes. And thus you will get less opportunity to use the murder knife, and do similar things. Also, they ditched the forging of your personality that you could do with Hawke, choosing "red" answers made you more ruthless in DA2. In DA:I the Inquisitor seems more neutral.
If one want to be truly vicious, you'll have to play SWTOR.