I think that something people often forget in "free speech" discussions about hateful and dangerous people like Kiwi Farms is that while deplatforming someone can be seen as an evil, killing or imprisoning them certainly would be worse. When you have an organization that promotes violence like the Christchurch shooter, when they begin to use violence in the real world, you will have to (a) kill or imprison them to stop them from hurting others, and/or (b) allow them to kill and injure others.
If you see deplatforming people as a preventative measure to keep from having to kill and imprison them (not to mention the harm they will do before you can stop them), then it seems like the morally best thing you can do for these haters.
Sadly our culture loves cleaning up things after the fact instead of deploying an ounce of prevention. This is how you end up trying to "harden" every possible target, like schools, installing metal detectors and buying children bulletproof backpacks, instead of just limiting guns. Not only is this expensive, impossible and ineffective, it makes everyone's life worse in innumerable ways.
Even before a killer is radicalized by KF, or a SWAT team is sent to someone's house, allowing this harassment to spread just makes the world a worse place to live. Everyone believes in some moderation, otherwise they would be fighting for the right of spammers to spam, and burying any platform they control under mountains of junk. Once we've established that everyone allows moderation in order for communications to function at all, we're just haggling over how many people we're willing to kill with lax moderation, and how nightmarish we're willing to let our "public squares" get before doing something.