A while back they took away his verified mark for being a douche, probably because they felt pressured to punish him somehow but didn't feel like he did anything that was clearly wrong enough to do so. Or maybe they just didn't have the balls. Regardless, they probably should have just prema'd him then.
No no no no no.
This is not how you handle people with horrible views. Pushing them away only validates their beliefs and solidifies their fan base further.
He wasn't banned for his right-wing doucheabg views, though, he was banned for all but asking his followers to dogpile on her. This wasn't even like the recent Notch incidents where people @'d him calling him an asshole, he responded defending himself rather than just absorbing it, and his followers dogpiled on. While Milo probably didn't start it, he very obviously went out of his way to draw attention to it and add fuel to the fire around her.
I mean, yeah, there are shitloads of other incidents of people from all across the political specturm doing this same shit and not getting punished (or just getting a slap on the wrist,) and the politics and celebrity of both of the major parties involved here are very clearly the reason why Twitter took action here and not in those other cases. But just because Twitter is biased and lazy about enforcing their rules doesn't mean they're wrong to actually do it when they should once in a while.
But yeah, while your point is generally right (because if we don't allow douchebags to state their douchebag opinions in public spaces then how do we know who the douchebags are?), when it comes to criticizing people in a space like Twitter there's a pretty big grey area between "legitimate expressions of opinion"/"participating in a discussion/argument" and "harassing/telling people to harass someone." This case was pretty clearly on the latter side, though.