I'd be one of the last people to defend Instagram/Meta/Zuck, but is that really your takeaway from this? Execution videos are absolutely against Instagram's TOS and I'm sure they want it on their platform even less than you.
If anything, Instagram's issue is they're too heavy on the robo-censorship. My friend posted a photo of him standing on one leg with a chair in his hands, making a goofy face, with the caption "I'm gonna hit you with this chair!" directed at no one in particular, and it was taken down for, quote: "encouraging violence and leading to risk of physical harm, or a direct threat to public safety." Appeal was rejected too, and the post had to be deleted.
This happens because they're trying to moderate against people posting execution videos. One slipped through the cracks seemingly, but it's not like they don't put a lot of effort into preventing that. It's pretty hard to do at Meta's scale where most of the earth's population is using their platforms.