A message board is not a source of any of those things. 8chan does not have a super-god who removes comments they don't like, there is no inherent leaning in the structure of the website. Any platform is liable to this kind of behavior, so what exactly does this achieve? If it isn't Cloudflare, it's self-hosted. If it isn't 8chan, it's Twitter, it's Facebook. There is no shortage of places where radicals could go to let their beliefs fester. What about removing hosting for 8chan resolves anything that you've mentioned?
Deplatforming 8chan isn't about limiting the ability of these people to kill large numbers (that would require effective gun control) - it's about removing a source of provocation, radicalisation, cheering, etc. for them both before and after.