Friend, I’m afraid you’re wrong.
Grimdark cannot happen unless one is dabbling in dark magics/horror terror stuff. Like Rose was. Rose went grimdark because of her communication with the outer rings, her obsession and ensuing use of horribly dark magic, and the trigger, her mother dying. It’s been speculated that it’s something that was specific to herself, because of the game shenanigans.
Eridan did not have either the horrorterrors, or the dark magic. He was angry, and the two panels are a styling choice. He dabbled in white magic, in ‘science.’ Not in dark. Feferi would be a closer choice for someone turning grimdark, since she was actually in communication with the horrorterrors.
Another note: Grimdarkness is not a light switch. you can’t turn it on and off. Rose’s grimdark stayed with her until she died and then rose (haha) again, due to her god tier statues.
While the subject of Eridan’s ‘self defense’ is still, erm, debateble if we’re being generous to him, it’s certain that he did not, and could not, go grimdark.
And I assure you that he could and he did. Why? Because he was ghosting his other classpect - Sylph of Rage. It’s not connection to Horrorterrors that warrants grimdarkness, it’s unleashing the uncontrollable inner rage that has been triggered by a stressful event.
He keeps flickering between Prince of Hope and Sylph of Rage further on.
That’s why Jade who has no connection to Horrorterrors whatsoever goes grimbark because she holds big amounts of aggression inside herself thanks to being prototyped with Bec who is perfectly capable of it.
I’ll go even further and say that Gamzee’s “Hero/Murder mode” is also a sign of grimdarkness.
His face is literally black under the make up and hair. That’s the most GRIM color of DARKNESS you can get.
Rose’s grimdarkness is triggered by Horrorterrors and that’s true! But in her case this is the only way she could achieve it, because she, as a player who has no connection to Rage aspect whatsoever, needed external help with it. Horroterrors are the best beings in the universe she could come to. Why?
Because Horrorterrors are alternate Gamzees. That’s why we never see alternate Gamzees in dreambubbles or in the army Tavros has gathered.
Their blood is exactly the same color as Gamzee’s.
And yes, you can switch on and off grimdarkness. It’s called switching on and off your brain. Rose switched off her brain by dying, Vriska did it for Jade by putting her to sleep.
Gamzee simply got calmed the fuck down by the most calming touch in Paradox Space. Considering the fact his brain is taken over by Lil Cal’s influence, this means it was removed from his mind because Caliborn can’t stand fluff.
There are no accidents or stylistic choices for the sake of being decorative in Homestuck, friend. It’s not that kind of comic.