What we've been taught about the Force is that there's a light side and a dark side. A Force user taps into one side or the other; not both at the same time. And each side seemingly has a few specific corresponding powers/abilities. Light side users (like the Jedi) don't appear to shoot lightning from their hands, and dark side users (like the Sith) don't seem to use the Jedi mind trick. I can't imagine a Jedi getting yellow Sith eyes, and I can't imagine a Sith getting a Force ghost.
And that's where we get into the idea of "balance of the Force." It's the balance between light and dark. For example, one of the theories about why the Sith developed the "rule of two" is because balancing the light of the Jedi (who used to always have many members) would mean that the fewer Sith there are, the more powerful their dark powers would be. If we extend that theory, it could be why Luke was so powerful (without much training) in the OT; he was one of the only Jedi left to balance out the Sith (Palps and Vader).
Anyway, I don't think light side users can tap into dark side powers, or vice versa. Once Anakin crossed over to the dark side, for example, he changed (yellow eyes being the visual cue). Maybe he wasn't able to summon light side abilities (like the Jedi mind trick) anymore. So, the hypothetical evil Force ghost wouldn't be able to exist. But that's why one of the Jedi's responsibilities seems to be helping maintain that balance of the Force: to prevent a drastic and dangerous shifting.
Jedi have to be careful not to go too far, because it would lead them to fall to the dark side. If a Jedi were to abuse the power of being a Force ghost, it could presumably open the door for a balancing dark-side power surge. As Luke explained in TLJ: "powerful light, powerful darkness." And as Snoke put it: "darkness rises, and light to meet it." So, the reverse would be true as well.
This "distinction" is much clearer in Wookiepedia/EU/books than it is on screen in the OT though. A few examples:
- Luke uses the same force-choke-people-to-death thing that is Vader's trademark force ability. (More than bad cataracts or lightning, you would think that using the force to murder someone would be THE dark-side-only aspect.)
- Luke does the same "force-texting" thing that Vader does (Luke contacts Leia, letting her know he's in danger and she says "I know where Luke is" under Bespin) and in the same way.
- Yoda can move rocks with his mind, Vader can move lumps of machinery with his mind.
- Vader can sense Ben's arrival on the Death Star, Luke in his x-wing and in the Shuttle in ROTJ, just as it's clear that Ben senses Vader on the Death Star and Luke senses Vader's presence on the Endor moon.
The mind trick thing ("light side only") is not really a fair assessment because Vader is so powerful in the OT he really has no need to use a mind trick for deception (which is all it amounts to) versus Ben who is on the run and in a position of great weakness. And Motti sarcastically mentions "clairvoyance" to Vader before he's choked - which at least vaguely suggests that Vader has indicated he has "mind tricks" that can help the Empire (that he hasn't delivered on.)
Even the dark/light "morals" aspects get super murky: Luke slaughters a metric ton of Jabba's henchmen (who don't even work for the Empire, just crooks,) just as Vader kills (though far less) rebels or his own officers. Ben's highly skilled at mental deception entering Mos Eisley on the Death Star, but CHOOSES to literally dismember Evazan and Walrusman because they are drunk and aggressive.
The ST and Disney continues this ambivalent dark/light equivalence: even if it doesn't come from his fingers, Yoda's ghost wields lightning like Palps does (more visually related to nature, but the result is the same - a force user producing lightning to destructive effect.) Vader's going postal in RO is not much dofferent Luke going postal in ROTJ.
Even stuff like non-intervention of force ghosts rules are vague from OT to ST: Ben's ghost refuses to help Luke even as he's about to fall to his death beneath Bespin, despite Luke BEGGING him to come. Yet Yoda's ghost randomly shows up in TLJ to "help" destroy the tree - both fairly pivotal moments for Luke for different reasons, but you'd think the ESB example would be the better intervention, even if both are technically "interfering" in Luke's trajectory.
I get that huge effort has been expended over the decades to try to build this lore (the construction of light/dark rules reminds me of the problems in creating "Aurebesh" language from what was seen onscreen in brief snippets in ESB and ROTJ) and massive numbers of publications have nitpicked stuff to set up "rules" but as presented in the OT it really doesn't present as that way.
It becomes almost impossible to argue against it all because there are hundreds of books, comics, shows, etc that have embraced this stuff - but all I go off is "what was filmed for the OT?"
There most certainly is a light and dark side in the OT, but its more just two approaches to the same thing with murky rules, more mixed together and the same thing that can flip-flop quickly, which to me is what made it so powerful and mysterious. Just like our own decisions that can turn on a dime in pivotal moments. It's this distinct lining up of light side and dark side entities and attributes that kind of bothers me. A drone can be a child's toy, an amazing tool for marine biologists, or can wipe out a village.
To me, a person can no more easily bring balance to the force than bring balance to our planet. To paraphrase the old saying, interpretations of the force are like *ssholes - everyone's got one.
So much of that EU stuff is muddled anyway - the ROTJ novel and many other EU sources attempt to say that Luke just burns Vader's suit (ie Anakin's body disappears like Yoda's did,) yet it's clear Anakin's body stays after he's died onscreen and the ROTJ screenplay says Luke sets "a torch to the logs stacked under a funeral pyre where his father's body lies." But no, that suit is empty now, folks (why would Luke transport an empty suit to a
massive funeral pyre and emotionally watch it burn?).
This is the kind of random and mysterious stuff I LIKE, and also shows how so many of these iron-clad "rules" ignore what was the intention or even contradicts what was onscreen.