My opinion, it is not overly gruesome. It is sadistic in a psychological way, not in a physical way. The story line: about a paid killer who is looking to kill to retrieve money stolen during a gone-bad drug deal.
In fact, when you think back on it, there's virtually no one actually killed on camera except that deputy near the beginning. In every other case I can think of, it happens offscreen. What you get a lot of is the prelude or the aftermath of violence and not the violence itself. And the most terrifying scene in the movie, with the store clerk, doesn't have anyone getting hurt at all.