Followup to my last comment....
Sometimes the qualifier "gratuitous" is used to describe when writers and directors choose to utilize violence in a work when they could have written a given scene or whole story in a different way. I.e., it's not used to refer to a lack of reasons for the violence in the story so much as a commentary on a style of storytelling. Could X tale have been told with less violence–or less focus on violence?
I see problems with this though. It just becomes a game of what if. what if there was less violence could just as well be less romance, sex, anything really. I have no doubt that any scene in any book or movie could be written or filmed with less violence are anything else for that matter. In this instance it seems more a complaint that the scene wasn't done the way the reader/viewer thought it should have been done, a back seat author if you will.
My example from a much earlier comment: A scene in which some sadistic villain tortures our MC or someone the MC loves. These scenes bother me greatly, especially when the author seems to be overtly trying to push my buttons by including the scene or extending the torture scene for a much longer stretch than necessary. The issue is a little like whether to describe a sexual encounter in great detail or fade to black. (But the question there would concern gratuitous sex rather than violence.) Either approach might be supported by the story and relevant to the story. I can easily believe in the existence of sadistic villains (or honest-to-goodness vibrant sexual encounters.) But are these detailed excursions gratuitous? That's a question we can debate forever, probably.
Honestly I think that when such things become overt then I see it more as a failure of the author and not so much violence or what have you.
One thing though I'm using the definition of gratuitous as being without apparent reason, cause, or justification.
Based off that it would seem that gratuitous violence is more like a meteor randomly crushing a character.
This brings up something that I have pondered. Isn't the goal of an author to push buttons? I would argue that it is their primary goal otherwise one would read a tv repair manual. Don't we want readers emotionally involved in our stories? I'm not saying leave the reader emotionally catatonic but some degree of button push is necessary. I think the trick is not letting the reader realize it.
I see violence, be it on the more extreme end or even the suggestion of violence, to be useful. Its a tool the same way romance is and they each have a place in the toolbox.