See, I agree, but there's still plenty of people who think the point of RP is to do whatever you want, regardless of the consequences.
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)?
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.
I can see in an AL game where you're forced to play with unknown strangers, that could suddenly be an issue, but I feel like AL games are a separate issue and should have better "safety rails" and other maybe-clunky safeguards than home games.
Interestingly I've absolutely never seen that kind of attitude from a genuinely new-to-RPGs player, or anyone coming in having watched Actual Plays or listened to podcasts or the like.
And it's also very hard for many people to speak up about what's bothering them, especially against someone who is louder or more forceful--or who is otherwise considered (rightly or wrongly) to be a friend.
Definitely!
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.
I will say it's problematic if the DM is going to be hostile to the use of the X-card but that's a DM problem really, and likely someone you shouldn't be playing with.