Eridan could have killed the lowbloods at any point before Hivebent, simply by inaction (not slaying as many lusi for Pacifist Princess Feferi while she cuddled her cuttlefish). He specifically knew this, but instead continued to commission over-the-top doomsday devices he didn't expect to work (cooperating with a landdweller to do so).
All the trolls were silly preteens trying to figure out their roles in life. As seadwelling royalty, Eridan was supposed to despise landdwellers, and he *failed*. He liked and respected his "inferiors" on the hemospectrum (Karkat and Kanaya for example). He did his best to cooperate in Sgrub, begging the others to visit his land. Maybe he was an idiot to aggro the angels, but I suspect not... either way, he was fighting for the team and hoping for help. He even tried to help Jade, a human!
Does anyone ever thank him for defying his blood caste? Not on screen. We can only hope that Karkat did at some point, since they seemed to have an actual friendship, and Karkat is cool like that.
Now, I'm not saying Feferi was wrong to break up with him. Obviously she had the right to, and I'm sure it was hard to be his moirrail. The guy was an emotional train wreck! However, she doesn't seem to appreciate all the lusus-killing he did. As a troll empress I'm sure she was capable of killing to feed her lusus, probably a lot more capable than Eridan. But she never had to before, we don't know if she ever did, and she seems to love animals! I'm just saying Eridan (the supposedly genocidal) had been doing her a major favor for years, and she acts like the whole relationship was a burden on only her.
Does that justify him murdering her? Not at all. And he didn't.
Eridan dueled Sollux in the land of brains and pitchforks. I don't think we know much about the circumstances of this, except that Eridan lost. This seems like perfectly reasonable behavior for two trolls fighting over a matesprit. Eridan presumably accepts this and continues to play Sgrub, instead of murdering Sollux in his sleep or something.
Later, after Sgrub, on the meteor, Kanaya tries to Return to the core. Eridan approaches Fef. Having acquired vast magical powers of Hope, he is pursuing a plan to *actually do something*. DIPLOMACY! With Jack, so he's kinda doomed, but his heart is in the right place. The trolls are literally waiting to die, they have no plan and no solidarity left. Eridan doesn't even go off by himself, he tries to talk Feferi into coming with. He's not trying to save himself. He's trying to reunite the troll royalty to officially negotiate a surrender, sparing the trolls a guaranteed death. (That's not conjecture, he says as much).
He does an admirable job at ignoring the mustard blood heckler as he presents the plan. Feferi (fortunately) vetoes the plan. She does so with a complete lack of diplomacy, including forbidding a *sea-dweller* from using *fish puns*, but hey. She's the Empress.
Sollux butts in again to say "ii 2hould have killed you on lobaf when ii had the chance." Followed by ATTACKING. He even powers up first.
Um, what the hell was Eridan supposed to do? He's getting assaulted with deadly intent by a lowblood simply for *suggesting* a plan. He didn't even say he would defy Feferi, Sollux didn't give him a chance. Eridan's response is plain self defense, fully justified under human OR Alternian customs. Frankly, proper Alternian behavior would surely have been for him to kill Sollux far earlier.
Notably, he doesn't even kill Sollux in self defense. (Maybe halfway? That part is weird.) He stands there as Feferi checks Sollux, and then... attacks Eridan with a deadly weapon. Again, Eridan responds with self defense. He looks grim, and who wouldn't. His ex moirrail, Empress, and unrequited love is about to stab him for knocking a lowblood unconscious. This is possibly when he snaps.
Destroying the matriorb is impossible to defend, except that it's probably a moot point. The Empress is dead, Kanaya's clearly about to attack him too, basically everyone is trying to murder him. The situation is completely hopeless so he finally, for the first and last time, listens to his blood. Whether he was justified in killing Kanaya depends on whether she was about to attack him before he destroyed the orb.
Keep in mind, I'm arguing using human morals. I really doubt that Alternian law would judge anything he did as wrong. The only questionable matter was Feferi's death. On the one hand she was the Empress. On the other hand it was self defense, and she was only one rung higher.
Karkat doesn't attack Eridan, so Eridan doesn't attack him.
Cronus is probably a fine troll but gog damn is he a terrible person. I see him as people's unfair perception of Eridan, personified. He even says "Nyeh"!