There is a huge difference between "livestreaming a shooting on facebook", and writing a 200 page manifesto about how /pol/ redpilled you. I guess all 200 pages were just "banter". The guy practically wrote a book on being radicalized by /pol/, and that to you, is the same as opening up Facebook and clicking the "record" button.
Those things being hugely different is just your opinion, not universally accepted truth.
This doesn't absolve 4chan, but rather damns Facebook.
I thought the same the first time I heard about a livestream murder on FB. I thought: I'd shut that feature off instantly if I were in charge. No way I could live with my conscience if someone used my platform for murder attention.