I mean, yeah, you do have complete freedom to choose what to write about! And in the end, if you write about something that someone thinks is monstrous, they also have the complete freedom to express that!
Though I do think it's rude and unproductive to go to a creator and complain about the content of their work -- going up to someone and saying "YOU SHOULDN'T HAVE [X] IN YOUR WORK" doesn't strike me as a particularly persuasive approach anyway. But for someone to privately message me saying "the way you use [x] in your work is troubling me, I wanted to ask if you intended these implications" or if someone wanted to write a public review or complain on their own blog that "this comic has an awful message that encourages [X]" -- like, those aren't inappropriate reactions for a reader to have!
I think zeroing in on the specific content misses the point a little bit. There are certainly ways you can use guns, punching, bombs, and zombies to say something really troubling. There are ways you can use spanking to say something important about your story or characters or world. The idea that you should NEVER have [X] content in a comic feels very Comics Code Authority to me and I don't agree -- but the conclusion of "you shouldn't restrict any content THEREFORE no content can be bad" doesn't follow, and I don't agree with that either.
Our work isn't restrained by reality, but it's also never completely contextless. What we say about our fantasy worlds -- and which parts of reality we want to escape from -- inevitably says something about how we see reality. No editor is going to stop a webcomic creator from wishing for a troubling reality, but people who read the comic might still object to it!