It's always a "fallacy" because the fallacy points out that the "next step" is not an inevitable consequence of the step it's arguing against. An assault weapons ban, to follow this example, will not magically manifest a handgun ban by its very existence, and it is entirely plausible that a handgun ban will still face tremendous resistance.

That the "next step" can be someone's eventual goal or desire is irrelevant to the "fallacy" part.

Not only not every, but in fact “slippery slope” rarely is a fallacy.