If kill somebody with an illegally owned firearm in self-defense, will I get charged with murder?

No.

That the gun was illegally owned has zero bearing on whether the shooting was legitimate self-defense or murder.

If the shooting was legitimate self defense then it was self defense, and that the gun was illegally owned does not change that.

You might, of course, still be criminally charged with illegal possession of a gun.

Richard

If the killing is deemed justified, a murder conviction would not stand. However, you could be successfully prosecuted on charges of having an illegal firearm.

if it really was self defense no.

but they will still charge you for illegally owning that gun.

not likley

IF it was legitimate self defense

but you might have a weapons charge