Remember, that both the far left and the far right are dangerous. For all the talk currently about fascism, it was Communism in the 20th century that produced the most death, suffering, and misery.
The idea that regimes claiming to be Communist or even Socialist killed many does not lend credibility to your claim that the far-left is dangerous; you seem to be taking issue more with the tactics that were employed, and furthermore teetering on the edge of a 'golden middle' fallacy - of course the extremes are dangerous, we should just stay in the middle which has always been fine, right?
Centrism has this strange tendency to throw out valid complaints from either "side" (if we really must classify political thought into left and right) in favor of maintaining the status quo. Centrism supports capitalism, which is why as a Communist I'm against it.
No. Antifascists primarily protect those unprotected by the state from being assaulted by those claiming the "supreme white master race". We protect the democracy because fascists will use the institutions of democracy to dismantle it - look at Hitler for what already happened, and look at Breitbart for what happens now. What Steve Bannon is calling for is, essentially, the destruction of the state.
There is no worth in trying to discuss with outright fascists, they will not stop unless they are stopped. And for the most radical of the bunch, like Spencer, yes, this includes violent solutions.