In quite a few of my games, I have a “dark” theme. As mentioned above, whenever I’m making a game, I’m not really thinking “how much is too much”, I’m just thinking about what I want to express, and how I want to express it.
There are times where violence might not add anything to a creation, in this case it might be better to be removed, minimised, or even just hinted, as it shrinks the number of potential users (mainly young people). However that’s up to the artist. Do they want to just express themselves? Do they want more users? Or both?