When both Jade and Eridan went grimdark, they were violent and hostile (even more for Eridan) but when Rose went Grimdark, she just became slightly more jaded. Do you think if another Light player would turn grimdark, their personality will remain the same, just like Rose?

See, the thing here is mistaking Grimdarkness and Grimbarkness, and thinking Eridan went Grimdark in the first place. There’s a HUGE difference between these three states.

Let’s begin with Rose, and define Grimdarkness as a state in which you’ve been Corrupted by Horrorterror Influence. After she enters this state, Rose gains Eldritch powers, is only able to speak in the Festertongues of the Horrorterrors (Those Stubborn Throes), and becomes single-minded in her very emotional quest to take down Bec Noir for killing her Mother. Her skin literally becomes grey, she has an aura of darkness surrounding her, and brings with her obscurity, pink lightning and black ichor rain.

Now for Jade, with Grimbarkness, there’s absolutely no influence of the Horrorterrors. This is a state in which Jade’s conscious, rational mind is being pushed back while her more animalistic instincts are brought out. She’s not being fully made into a mindless drone, as much as she’s being swayed to Condy’s side, and forced to have all that’s bad within her come out. Frustration at her relationship and other people’s inaction, rage, instinct, she becomes a primal embodiment of all the bad things Jade has deep within her. She can still speak, her First Guardian/Space powers aren’t suppressed in the slightest, and her skin literally becomes grey as well.

Finally, Eridan is not being influenced by anything at all. The darker grey tone of his skin is a stylistic choice through his Murder Spree. There’s no Mind Control, there’s no Swaying, there isn’t even a power-up of his abilities- The White Hope Energy is heavily implied to be coming from Eridan and not his Wand, similar to Rose’s Magic. These are intrinsic abilities that he learned to rely on, as a Hope Player, when he dumped his previous attitude of no-Magic (Even though it’s still a ‘science wand’). He channels his energy through it, and does what he wants in that moment. He tries to offer Feferi one last chance to join him as he’s about to pretty much beg Bec Noir to let him join his side and survive in exchange of telling him where everyone else is, before proceeding to KO and Murder anyone who stands in his way when he sees no one wants to see things like he’s seeing them. He’s in a desperate state, he has lost Hope, but at the end of the day, those violent tendencies are a part of who he is and he shows absolutely no regret about his actions until after he’s dead.

In short, Grimdarkness is a state in which someone is being corrupted by the Horrorterrors, with an added power-up and a close tie to Void, and Rose shows her will being amplified rather than suppressed- She is given the tools to go fight Bec Noir, arguably, even while being corruption it’s a ‘boon’ by them. Grimbarkness is a state exclusive to Jade because she’s a furry, when the more rational, kind part of her is being repressed and all that’s bad but already there comes out, with a sway to Condy’s side. Eridan is not Grimdark, nor Grimbark when he goes on his rampage, and is just being a huge asshole with too much power for his and everyone’s well being.