He blew up the Matriorb because he'd pretty much gone off the deep end (do you really think under normal circumstances he'd be able to kill Fef and not bat an eyelash? Shooting her was the first clue he'd completely flipped his shit). I think if he'd lived a little longer afterwards, he'd have had a breakdown over killing her. The reason he went to face off with Vriska was because thanks to her, Jack was unstoppable.
I also hate how no one even mentions him after he's killed. Yes, I get why Kanaya killed him, but I mean, they were close friends before, wouldn't she at least have a moment wondering why he did what he did, or I dunno, just say "I wish it didn't have to come to this?" It's little things like that that make the whole "Eridan is an irredeemable bad guy" thing seem kind of contrived.
Ugh, I'm ranting. But who cares?