In a world with devils in it, this is a very logical and reasonable thing to consider. And why shouldn’t the Ghost be a devil? It’s already supernatural to have a Ghost appear; why not go an additional step and have a devil ghost?
In fact, what if, even if what the ghost says is true (and we have to assume it is, based on Claudius’ soliloquy) it were still a devil? After all, this play does end in a TOTAL tragedy. The entire Danish court is murdered. Every single one. As a diabolical plan, this one is pretty good.
Maybe he doesn’t end up with everyone going to hell – (it’s possible that Hamlet and Laertes escape the (rending?) flames due to their exchange of forgiveness but otherwise, the devils would seem to have racked up a nice list here.
1) The old king Hamlet (all his sins upon his head)
2) Polonius? (This one’s a toss-up. We have no real evidence of his sins.)
3) Ophelia (drowning yourself being a one-way ticket to hell.)
4) Gertrude (assuming she had a hand in at least one of the things going on around here- adultery, maybe? Accessory to murder, possibly? Standing there watching Ophelia drown herself maybe?
5) Rosencrantz and Guildenstern (just for being toadies, I’d say. Though maybe they get a second to absolve themselves before they get beheaded.)
6) Claudius (big score there)
In terms of creating mischief, this is great devil’s work. In terms of upping the population of hell, it’s also pretty damn good.