Why would someone create a weapon that can not be controlled?

You wouldn't .  But,  some think that the Chinese intentionally released the Corona virus, wrecking their economy in the process.  The Wuhan regional officials did conceal the outbreak for a while and that's bad.  
The weapon being deployed by the right is ignorance - and, that's easier to control 

Because they were still working on the "controllable" part, when it got away from them. It wasn't ready for prime time yet.

Somebody always thinks they have control. In World War I they used poison gas, but if you weren't careful it would get your own troops.

If you're talking about COVID-19, it's not a weapon. It's a natural disease.

Noone but the Joker whould do that

That's the nature of the beast......when creating something new.  You don't really know how it will react.   

You try for  A, B, or C......and suddenly, you get D, E , and F that you didn't count on.   Next thing you know.......the genie is out of the bottle......and there's no putting it back. 

Did you know that  dynamite was actually an accidental discovery. ? 

Yes, Alfred Nobel and his brother were in their lab, working on something else, when his brother blew the lab to kingdom come. 

It was a mistake.......but they (inadvertently,) discovered a formula for a highly explosive new gun power. 

Alfred, naively believed that he could put it to GOOD uses.......it was used extensively in building the Panama canal,  to blow out huge sections and obstacles. 

But of course, the MILITARY immediately took a keen interest in it.......and the rest, as they say, is history. 

Mad scientist kind of stuff. We don't have any.

Insecurities and Hubris at alternating intervals. 

Nazi party has continually referred to "culling the herd", and "natural selection", through social policies.  Motive, ideology and means. They have no obligation to think differently, or respect their predecessors. 

Perhaps, another vain attempt at trying to warn the public, but they eliminated the policies and programs that guarantee survival, as well as the means to sustain private assets. They can't pay what people paid for in former years, and they cannot afford to guarantee what the population was promised before their party found a way into government. Any effort to restore American legal standards, and private assets will be met with an attack on the people and economy. You were warned repeatedly and chose to ignore it.

The number of sick and dead would be considerably lower with former American ideology.  Even presenting all the evidence, no one would comprehend it.

They might if they were desperate.

They might if the lack of control was outweighed by the effectiveness of the weapon. (See, for example, the use of war elephants.)