Eridan is best

About the part when eridan kills everyone: Fef charged at him with her weapon, he hadn't provoked her at all. Kanaya was also thinking about attacking him, and also eventually did.
Don't forget that Sollux pretty much bought this on as he was the one to wind Eridan up to this point, and you could blame some of what happened here on Eridan's uncontrollable anger that resulted.
He really did want to do his best, he asked Fef to join him in joining Jack because that's what he believed was best - he was just misguided.