I can see why the fascist left has to use violence to prevent conservative speakers like Ann Coulter and Ben Shapiro from appearing on college campuses.  It's pretty clear that articulate speakers who can defend their arguments are a threat to college students who have never heard strong, conservative arguments and that might cause some of them to leave the plantation.  But I don't understand why they are so afraid of Nazis and the klan.

People who spout Nazi and KKK rhetoric are pathetic losers.  They are the perfect people to put on stage and then mock them and show how foolish they are.  But the left doesn't do this, instead they  prevent them from speaking at all.  Why are they afraid of "sunlight being the best disinfectant"?

They can't really believe that Nazis, given a chance to speak freely, would actually convert people to their way of thinking, can they?

Are they afraid that having to debate Nazis would mean having to demonstrate that their ideas are better (or actually any different)?

Or do they simply want to equate any conservative with Nazis and having only 50 Nazis show up to an event would make it obvious that they are shunned by the right as much as the left?

Well in part, it is because they want to use the Nazi label to shut down speech from conservatives in general.

Deliberately preventing a marketplace of ideas isn't often an explicit goal.

Chasing scarce virtue signalling opportunities is one part. Maintaining the fantasy of being victims punching up against a big dog (not establishment punching down on a fringe) is another.

Nazis don't have any interest in debating, they want to do other things to people they dislike.

I don't think it's just conservatives...I think they want to gag anyone who is unwilling to eat their shit, exactly the way it comes out of the spout.

The left can't debate Nazis because they AGREE on most things, save for the whole "racial purity" thing.

Nazi's are leftist Socialists, after all.

The klan and the nazi are just a way to smear Trump and drive a wedge between him and the republicans.

nazi to them = anyone not left.
I dont know if you guys know who Flecca (Flecca's Talks i think) is but he does a youtube channel where he talks to these idiots at their rally's just to see what they say.
He asked them how they deal with nazi's. They blatantly said you have to kill them.
Leftist want to kill nazi's while claiming to be full on lenin communists.
You cannot talk with them, you cant rationalize them they are miles past that part now and just want to beat your face in or kill you.

They just like hurting people. And they are probably really bummed they aren't shoving people into ovens yet. It's like that. 

This.

nazis are national socialists and the antifags are international socialists. It is about competition. There can be only one socialist power.
That was one of the main reasons for WW2.

They don't care about free speech, it's about their candidate losing, and whatever perceived benefits, freebies or visions of utopia that came with it; you can't debate against losing free stuff. This is just the latest distraction and they will eventually move on to some other issue or thing to destroy and a different name for people that doesn't agree with them.