Propaganda is a weapon. In an ideal world we wouldn't need weapons at all. But when an enemy is using a weapon against you--whether it is a spear, crossbow, gun, cannon, tank, nuke, or propaganda--you need to do one of two things:

1. Deploy a defensive solution to neutralize the weapon
2. Use the weapon yourself to even the battlefield

History is littered with examples of conflicts that were lost due to the inability to adapt to contemporary warfare. I've not aware of any feasible solutions to defend against propaganda. Many solutions would be deemed autocratic themselves (e.g., banning the sources of the propaganda, such as Fox News).

The "high road" solution would be to educate and deprogram the affected individuals. But deprogramming 10s of millions of people is a monumental undertaking, would likely take generations, and would be met with resistance the entire way. The logical conclusion for someone looking to improve the situation in their lifetime is to pursue (2) and deploy counter propaganda.

Left wing fascism is NOT the answer to right wing fascism

I get what you are trying to say, though I think it would be more accurate to phrase it as "left wing authoritarianism is not the answer to fascism" because "left wing fascism" is an oxymoron.

That said, I don't think the use of propaganda by the left in a battle against the forces of chaos would inevitably lead to a left wing autocracy. But even if there was a chance it did, I would still prefer it to the certainty of a fascist takeover that comes from completely neglecting one of the war fronts. I imagine we would both agree that right wing propaganda has done massive damage to our society. Why would we let that continue unchecked?