Violated a basic rule of good, decent, honest horror (mild spoiler)

yes. as father myself i totally agree. i cant stand this. Same as in "It". as a father this is so hard to watch...

Yes, the only way to kill a child is in a way that doesn't make the audience feel. For me that is worse, because it desensitizes us to how horrible a murder of a child really is. You use the words decent and horror together? That is called an oxymoron. The murder of the child here made you feel something. Isn't that what is supposed to happen? No wonder we are such a violent and sick society, because our movies have taught us that brutality against another person is a plot point that isn't supposed to make you feel horror and outrage. Murder and killing has become a fun thing, that doesn't give us any pause to understand exactly how we are supposed to feel. Your premise is ludicrous. I've never heard of such a rule and think it is something that people expect now just because everything in horror now is sanitized.

The creepiest part to me is that it was real in line with the rumors of Illumanti ritual murder. I'm pretty sure this kind of thing goes on in elite circles and the filmmakers were showing it.

The scene was originally more violent. Stephen King told Mike Flanagan during a screening.

"It was one of the only times he leaned over to me during the movie was when Tremblay got killed. He leaned over and he was like 'That's a little brutal isn't it?' I was like 'S*** I gotta go back, I gotta go back and edit this. I gotta pull stuff out.' And we did, we changed it.

I thought you were going to mention another rule of good horror (and hell, of good writing, full stop): your antagonist should be a credible threat.

Seriously, you can't possibly expect your audience to be frightened of them if your protagonist just effortlessly clobbers them in their every single confrontation; not once did it feel to me like Rose was anything more than a mild inconvenience for Abra.

I saw it opening weekend with a total of seven (I counted) people in the audience--all black except myself, oddly enough. It wasn't because there was terrible word of mouth that little Jacob Tremblay had a particularly disturbing death scene. I really can't explain why it flopped. I thought it was a lot more engaging than IT Chapter One (I didn't bother with Chapter Two) and that became one of the most successful horror films ever released. It should be noted both films contain graphic depictions of children being murdered, so your prescription is invalid.

"You just don't kill children in brutal ways. You just don't."
-Thats not a horror movie rule......

LOL sure bud.