My take on the moirail thing.

Mizzim has the truth of it - all trolls are potentially volatile, moirails are supposed to keep each other grounded and supported. Gamzee and Karkat are a great example of this.

The problem with Feferi and Eridan is that Eridan doesn't want her as a moirail, he wants her as a matesprit* and is being increasingly aggressive about it. And she's not interested. The fact that she shows any interest in Sollux (who, besides being a lisping nerd, is near the bottom of the hemospectrum) not only adds fuel to the fire of his rivalry with Sollux (no blackrom here, they simply want to kill eachother), it makes her respond poorly when Sollux is shot, which leads directly to her death.

* Honestly, Eridan seemed to just want anything with horns as a matesprit. He was a sad and lonely guy who only had his pride to cling to, but his pride was what made him so repulsive to everyone. The catch 22 made him ever more desperate and unhinged.

Nepata and Equius are much better moirails. Equius may be caste obsessed, but he really does care for her and wants what's best for her - it's just that he can't accept the thought of her flushing with someone of a lower caste, which is tricky because there's nobody on the entire meteor who shares a caste, so any relationship would be guilty of that. They protected each other, fought for each other, and tried to keep each other from self-destructing. In the end their focus on protecting each other was what put them in a position to be killed.