I don't like "alt-right" as it's more of a perjorative that's tossed freely around to discredit people than a meaningful description.
No, it's not a pejorative. It's a term that was adopted by white supremacists and nationalists.
In any event neither the left nor the right, insofar as such labels still have meaning, has a monopoly on this type of despicable behavior. Bully-boy tactics, ranging from using physical intimidation and mob-violence disruption all the way through to death threats and actual killing, are regrettably standard techniques for political extremists. They have been since Og disagreed with Thrag about who should lead the tribe in the mammoth hunt, were documented feature of organised politics since well before Roman times, and are still with us today. Neither the left, the old right or the "alt-right" invented them.
Quite true. Extreme right dictatorships are no different than extreme left dictatorships. Any government that use power (in any form) to oppress their citizenry is not a valid government.
If they find who made the death threats then they should suffer suitable judicial consequences. It is imbecilic, criminal and inexcusable behaviour.
Having said that, I am also appalled that Goff and Davidson have sided with people threatening violence and intimidation, rather than backing free speech. The correct response is to support free speech (even if you loathe the speaker and what they are saying) and then rebut them. In a liberal democracy, I would have hoped that politicians of all hues would have agreed on the importance of protecting free speech, and particularly political speech.
They don't have to provide or make available a platform for the speech though.