If you actually read my comment properly you'd notice that I explicitly said there should be a default ban list blocking that
> Either you yourself will have to watch the beheading video in order to know to ban it, or another group of users will need to tag it as 'disturbing' and you will need to ban 'disturbing' content'.
That's orthogonal problem; If you wanted to get rid of that entirely you'd have to pre-screen every content and that's just not feasible, nor any of the media does it aside from some automated filters on keywords.
Nor it is actual problem if you don't subscribe to people that post that; random nobody deciding to post that won't be featured in any feeds anyway because of how algorithms work so there is very little chance it will land on accident in someone's feed.
Of course someone could play the long con and get popular only to start publishing disturbing stuff, but none of current systems stop that and I'm not sure you even could.