Facebook actually attempts to remove illegal and offensive content, though. Voat, Gab, 8chan, etc. pride themselves on deleting nothing. Nobody, Facebook included, can predict a live video of a shooting. You can't pre-emptively block a video. You can, however, see a pattern of content being posted and decide whether to ignore or act on it. This is what Reddit does when they ban subs like FPH, The_Donald, CoonTown, etc. They are actively taking steps to prevent such things. Your "free speech" sites don't do that, they let it roll and are consequently tolerant of such activities. I won't go so far as to say they advocate it (though I honestly think they do) but they actively defend its presence.
actually attempts to remove illegal and offensive content
Andrew Torba, owner of gab, posts frequently how often he works with the FBI and other law enforcement to take down posts and turn out users. He also posts how he will not comply with totalitarian governments like the UK to identify users for silly words. You have the absolute wrong impression of that site.
Given that say the Christchurch shooter live streamed on Facebook for 17 minutes but FB had no repercussions but Gab and 8chan did, you gotta really seriously think about who is dictating what you see on the web. Why does Facebook's wikipedia page not say they are a haven for mass shooters live streaming murders?