The actual fuck
Why are we blaming Equius for Nepeta’s death? He did everything in his power to prevent it! He told her to stay hidden while he dealt with Gamzee, and he barricaded her hiding place! If it had occurred to him that she might get in trouble by getting into the airvents, you can bet your last boondollar he’d have welded them shut or something. It’s not Equius’s fault that Nepeta is a gazillion times more devious than he is, and he certainly never intended to die in front of his moirail. And even then, she made the decision to attempt to avenge him; she probably could have crept away through the vents again without attracting Gamzee’s notice.
If we’re going to blame Equius for Nepeta’s death, then Karkat does not get a “clean slate.” It was in his power to stop Gamzee from the very first creepy pesterlog - Gamzee was constantly asking Karkat to come stop him - and he instead flailed uselessly around the lab for a few hours and, oh yeah that’s right, ordered Equius to kill Gamzee.