The Hamlet Project

Never believe what? That he lives?

Is he telling Hamlet never to believe it?

Or The Unsatisfied who look pale and tremble at this act?

Is it a way to say They’ll never believe it?

Is that why he’s denying Hamlet’s request to tell his story by threatening suicide?

Because they’ll never believe it anyway? May as well join Hamlet in the afterlife?

It’s weirdly oblique.
It’s not clear what the “it” refers to. It follows a request to tell a story to the Unsatisfied.
An action plan for suicide follows. It’s an odd bump in the road to Hamlet’s death.

Maybe it’s a request to never believe that Hamlet is dying?

Maybe Horatio thinks if Hamlet refuses to concede his death, he’ll bypass it entirely. Maybe he’s one of those “Believe it hard enough and you’ll make it come true” people.