So I guess this October I'll try to get a basic primer on Vriska up

Fey-child on Chapter 6
Yes he is an emotional mess, but so is vriska. The difference is we see why vriska is an emotional mess. We don't see what caused Eridan to be the way he is, and so naturally chalk it up to him being a dickhead. Here is an example. If you didn't see her conversation with John, where she apologises for everything, confesses self-hatred, and explains everything, etc, why would she be the way she is, a butch who crushes everyone else, emotionally, and apparently enjoys hurting those who can't fight back (tavros). But we do, so we understand her inner confusion.
We don't get that for Eridan. We don't get his reaction to murdering Feferi and kanaya. We don't see him break down and explain everything. We see him act an asshole. But how do we know that isn't, like it was with vriska, just him putting I on. That he thought he needed to be like he was, a genocidal maniac. I think he also suffered from low self-esteem, like tavros, but he became, instead of introverted and nervous, offensive and brash and not-giving-a-fuck-about-you. His social incompetence in relation to Feferi might be that he genuinely thought it wasn't a problem for her, that she liked hearing his problems. But when she began to pry, he became nervous and didn't want to look weak, so clammed up (I know, sorry). This is all speculating, and unfortunately hussie won't give him a chance to explain himself, as there just isn't the demand for it.

I also think you are dragging out the fact that he kills her, a bit much. He went to her offering what he thought was a good deal, Sollux attacked him, Eridan won, Feferi flipped and attacked him. He was defending himself from her. I think he got caught up in the moment, as vriska did with tavros, and just let loose.