Eridan is... Well, difficult for me to categorize.
On one hand, he wants to kill all the land dwellers. He's basically troll Hitler. But somehow, I don't hold that against him. He's also whiny, overdramatic and kind of annoying, but I still like him. He's like some kind of ineffectual sympathetic villain, technically trying to kill everyone and destroy everything but so bad at it that you feel sorry for him. I'm pretty sure this is a pretty common archetype, and TVTropes probably has a page on it.
I don't think Feferi is a bitch, or in any way mean or evil. Just kind of... callous. Flighty. Completely nonchalant about the whole troll race snuffing it. Constantly excited for no real reason. Is it evil to pretend to be someone's friend to keep him from killing most of the global population? I have no idea! But it somehow seems that she should be showing a bit more emotion at everyone dying and Glb'golyb getting prototyped and dumping Eridan.
They're pretty much polar opposites. I don't really know where the whole Feferi-hates-Eridan thing comes from. In actual canon, her ability to hate things has been shown to rival Tavros'.
Also, it seems that while I was typing this, the thread saw ten new replies regarding posts-per-page dynamics and descending shades. Huh.