You can't blame an actor for PLAYING THE ROLE AS WRITTEN. He Played Ronin exactly as the character was supposed to be. When everything you love is gone, and you are reduced to living like a hunted animal, well, gee, it kinda affects you. As stated, in Game of Thrones he not only had to act, but had to deliver in a made up language, he did.
When Khal Drogo was reduced to a vegetable, I was very saddened, and he conveyed being a lifeless husk incredibly well. Acting like a vegetable is not as easy as it sounds, try laying still, not moving, blinking, your breathing measured...now try it for hours. I Love Arnie's Conan, but am really looking forward to this since it will be the book Conan instead. Who knows, maybe we'll get a Groo movie some day.