I think it's to further enhance how Penelope's limited and skewed perspective at the start of the story led to erroneous judgment calls, as well as she hadn't really pivoted to thinking of the people as people and not just characters. Her logic at the time of choosing Eclis made sense from a purely player perspective. It just got twisted along the way as he turned out to be a very real and very traumatized person who already lost absolutely everything, put all his eggs in one basket (her), and then felt like he had it ripped out from under him again. Eclis is definitely a psycho, but it was Penelope's own actions that contributed to sending him down that spiral by finding him at his lowest, picking him back up, and continuously manipulating him. He wasn't a psycho in the game, after all, or at least that we saw. He's the foil to the prince, who she initially saw as a total psycho but warmed up to as she realized he was more reasonable and human than she'd initially believed.
Penelope is in a game simulator where she believed that in order to escape the game, she needs to romance someone to 100%– she chose Eckles to be that person. what she didnt realize is that reaching 100% didn't get her out of the game, but Eckles is too far gone to turn back. The point the author is trying to make is that 99% love (where Eckles is at rn) is not love, it's obsession and it can drive someone mad. If I remember correctly, the prince is somewhere around the early 80s–so his love for penelope hasn't reached obsession levels. If penelope hadn't romanced Eckles the way she did, perhaps he'd be a normal guy like the prince.
Now that I think, I guess it was doomed from the start. Had the meaning of the colors been revealed from the beginning maybe it wouldn't have come to this mess we have now. The game was pretty nasty to hide things from Penelope. She eventually uncovered why the characters behave the way they do, why she was in the game, etc but it's already pretty late and she has already suffered from much.

Why did the author make him even a romance-able character ?? Even gave him a sad backstory just to make him some psycho who wants to kill her makes no sense