i always thought of eridan going "grimdark" more of as him giving in to despair, cuz thats what a prince of hope would do, wouldnt he. rose went trough something similiar after she saw her mom die. when you get really, truly desperate you feel like the worlds shutting down and that theres only one way out

The thing is that, even though Rose felt sad about her mother, she wasn’t hopeless. She was filled her anger- He reaction to grief is to lash out irrationally, as seen in Game Over as well. She didn’t go Grimdark until she explicitly contacted the Horrorterrors, and then they preyed on her negative emotions to make her their pawn. Eridan just snapped and gave in to his murderous tendencies without any sort of external corruption, just his own despair.