>Is it? Would someone refusing to wear a mask, provably resulting in them asymptomatically infecting someone with COVID-19 be charged with negligent manslaughter?

It is not murder. There's a concept called "Negligent homicide," but even that is relatively well defined and requires intent to cause harm.

Don't want to get into the rabbit hole, and I am not in agreement with all the laws the US has, but the point is that "bad speech" is way harder to define than murder/homicide. Not only is it harder, it is arguably impossible to attain a good consensus on what it is.

PS: I don't agree with the premise that we should regulate speech, but if you were to want to do it it would most likely backfire or be counterproductive because of these reasons