See I think this is the big difference in perspective here.
Personally I think those people have fallen to the wayside of RPGs over the last 30+ years. Like, 25 years ago, were they around? Absolutely. To the point where eternal vigilance around new players to groups was required to ensure they weren't one.
But when was the last time you saw one in the flesh (including in VTT games and the like as "flesh" ofc)?
Me personally? Not in a while. Some of the people at my table have other tables they've played at and have had to deal with (and kick out) people like this within the last 5 years. (Including someone, apparently, whose preference for always evil orcs was just the cover for some particularly nasty IRL racism.)
But it also doesn't matter if I personally have witnessed it--I am a reclusive introvert with massive social anxiety who only plays with the same five people and have been for years. I've been playing with one of them since the 90s, even. I don't seek out new groups (if I lost my current group, I'd really have no comfortable way of getting a new one). Because I don't have a ton of personal reference, I have to listen and read what stories from other people, some of whom I know in real life, some of whom I know only from forums like this one.
And from that listening and reading to what other people say, I can say that there are a disturbing number of expletive-deleteds out there. Even if they're an extreme minority, well.. the entire phrase is
one bad apple spoils the bunch, not that one bad apple just makes for a bad game.
Because for me it's over 20 years. People like that are so much easier to detect now, because they almost always out themselves in their social media. And I'm not saying "Oh you must check their social media", because it doesn't get to that point, rather someone will know them, and they'll have seen their social media, and freaks like that love to signal that they're freaks. The last time we acquired a new player, it was easy to see he wasn't going to be a problem from his socials - and we met him beforehand, and he was a cool guy. The sort of crank who thinks it's cool to murder and eat an NPC (as per your example) is absolutely going to be signalling the heck out of that via social media in a variety of ways.
Well, that also assumes that people have social media (I don't, except for forums like this which are basically anonymous)
and pays attention to everyone else's social media. It also assumes that you actually know the person.
@Hussar apparently does lots of Adventure League. I've never played AL games, but I imagine that you don't get to know all that much about your players in advance of one of those sessions.
This
also assumes that people are always roleplaying themselves. I have no problems making a combat character while being mostly a pacifist in real life. Murder and cannibalism are terrible in real life, but as a GM, my favorite genre is horror and I would gladly use both as the actions of the bad guy (and I have). Lots of people play evil characters in RPGs as a way to let loose without worrying about the consequences, and I'm sure you wouldn't say that they're signalling
that in social media.
This is why X-cards or a similar mechanism are so important, and it's often a good idea as the DM/GM/Storyteller to invoke one yourself if you see something you don't like, esp. as it can break the ice on using them. Once there's a formalized mechanism, that really helps.
Safety tools are beyond useful and should be a necessity, or at least something that gets a paragraph or two in the core books for every system. Even in my close-knit group, we still stumble upon things that we didn't know would upset people.
But there
are people who, as you say, are going to be hostile to X-cards for reasons that range from "my group doesn't need it" to "that's woke crap" to "they just need to get over themselves and face their problems" to "why should I have to change my game for them."