What is the meaning of discomfortingly gratuitous even by slasher standards?

Answer 1

You seem to be talking about violence. By ordinary polite standards of behavior, violence is abhorrent and causes discomfort not just to the victim but to onlookers. A slasher movie is one that uses violence because some people seem to enjoy watching it. So you're talking about a different standard than polite society calls for. Gratuitous violence is the sort that is shown just for its own sake, not because (for example) it's a war movie and war is inevitably violent. So if you have a slasher movie, you're going to have gratuitous violence. If there's even more unnecessary violence than the low standard of slasher movies calls for, you'd describe it in the words that you quoted in your question.