Great breakdown from oomfie :) all I have to add is that I would never forgive myself if nobody pointed out that Eridan did not actually ever go grimdark, it's basically a fandom myth that he did, and that grimdarkness doesn't even make sense as an explanation for his murderous actions because grimdark Rose retained her existing moral code and was even capable of hanging out casually with John... she was just surrounded by a huge magical aura and could only speak in the language of the horrorterrors at the time. Grimdark doesn't mean insane or evil, the whole joke is that it's a very silly and basically superficial transformation that has no real effect other than making Rose look more Goffik.

Exactly like danya said, the complex and contradictory nature of Eridan's character is what makes him interesting, so making up (literally magical) reasons that he was never actually complex and contradictory to begin with just detract from his value rather than add to it.