The same and obvious inconsistent purpose for guns does not apply to fascism, which is why your example with guns is a poor example.

• If you are anti-fascist, you reject fascism.
• If you are anti-anti-fascist, you accept fascism.

Similarly,

• If you are anti-guns, you reject guns.
• If you are anti-anti-guns, you accept guns.

See how it works just as well as “negation” so long as you don’t attach an inconsistent purpose or meaning to what you’re negating?

You can certainly go ahead and assign inconsistency to antifa to make the point that anti-antifa is not equivalent to pro-fascism, but that really has nothing to do with the meaning of the anti- prefix.

So you can call it an anti-murder device.

Yes, but you can also call it a murder device. So when you say “anti-gun” and follow your logic we don’t know if you’ve meant you’re anti-defense or anti-murder. The ambiguity exists because of how you framed your example by attaching an inconsistent purpose for the gun.

The same ambiguity does not exist for anti-fascist.