There's actually almost no violence in Shutter Island at all. Sorry for the late response btw Nate--haven't logged on lately.
The movie is all about Dicaprio's character's repression of memories and trauma, and how he creates illusions for himself to cope--the illusions are portrayed literally, which is why the movie works so well. I highly recommend it.
EDIT: Although there isn't much violence, there are a handful of scenes that are extremely disturbing; without spoiling much storywise, you see dead bodies at Auschwitz, and dead children. I would say they are much more disturbing than the mindless violence in other Scorsese films. So in that sense, it might be a more challenging viewing...