2.) Although I'm a philosopher working in related areas and know plenty of people working on hate speech, I confine myself strictly to problems in metaethics and value-based decision making. So I don't draw the line. Generally, this is done by lawmakers and judges applying the law. For example, using Nazi symbols or publicly supporting wars of aggression is illegal in Germany.
3.) Judges and lawmakers.
> For instance, you mention conspiracy theories; do you think we should ban users who tweet about conspiracy theories?
Yes, for certain types of politically instrumentalized conspiracies that get intentionally amplified. The same for misinformation and fake news accounts, especially those run by state agencies like the FSB. But that's about moderation, not about illegal posts. Of course, any working social media need strong moderation. The lack of moderation is one of the reason why Twitter is such a horrible place - but there are other causes, it basically encourages toxic behavior and passive aggression due to its rules and their inconsequent and inconsistent enforcement.
However, moderation and blocking speech for legal reasons are two very different issues. I was suggesting that there is also a lot of illegal content on Twitter (according to local laws, not necessarily US law). When looking for #ukraine most notably open support of genocide, racist slurs, support for war crimes and the breaking of the Geneva Conventions such as displaying movies of POWs under duress, advocating torture, and supporting the bombing of civilians. There is also a high overlap with hate speech against the LGBT communities, mostly because many of the violently pro-Russian posters are also MAGA people from the US.
As for your doubts about what I see on Twitter, I took the liberty to quickly compile a short list of questionable and toxic accounts that pop up just by "exploring" for #ukraine Top and Latest (today, right now). To put this into context, you're right that the majority are pro-Ukraine. However, there is a danger of committing a fallacy of misleading two-sidedness in this case. Ukraine is the victim, Russia the aggressor. See my comment above about the German law. Supporting one or the other is not on a par, neither morally nor according to the law.
@Kanthan2030 @21WIRE @DravenNoctis @georgegalloway @WithyGrove @LanaAsanin @Jewels_MsJay @Qujuz @2cnlkh68 @THEENDGAMEWAR1 @MettsnBock @BowesChay @mazzenilsson @DasRgen4 @Judgenap @RealScottRitter @amitashi1985 @EmpireCide @Ri1_AAA @AdamStorm98
I'm not claiming that all of them are posting illegal content, but e.g. the last one wrote a post why a police officer should get the firing squad for being "woke" and a "lesbo". It's just the typical content you get on Twitter.