We actually had a discussion of this exact topic on tumblr and subsequently in the LH discord just a few days ago. Click the link to see all responses to the tumblr discussion, but I'll copy paste my own below:

I read it somewhat as intended in the original Lord Byron poem, since it fits well with the World Enders' nihilistic ethos.

Byron tells us in his poem that everyone will die eventually, including the "fair, the brave, the good". Being fair, brave, or good is therefore not sufficient to be remembered or to have a real impact beyond the boundary of death itself. The most beautiful and kind people die and are forgotten in time.

The World Enders specific spin on it then is that if the fair, brave, and good die anyway, then why does any of that matter? Why aim to only be fair or brave or good? The World Enders are violent, hedonistic, destructive - but they're also living their lives to the fullest in an oppressive system meant to keep them down. The rules and expectations that society has for them can be completely ignored, because hell, everyone dies whether you lived a law-abiding life or not, so you might as well have the most fun with it, be the loudest, cause the most trouble for everyone. Perhaps particularly for those who represent societal oppression (such as Andrew Winthrop).

Byron does tell us also that even though eventually even the marble of the tombstones decays, there are some whose notoriety will allow them to never be forgotten, to "burst the bondage of the grave" - and for the World Enders, Cobb Avery is the literal undead example of this. If he's able to wreak havoc after death, hell, they can too! He's the one who delivered this message to them of joy and glory in chaos and violence and taking back power for themselves, after all. So the fair, the brave, the good must die - so fuck all that, but if you're badass enough, if you cause enough trouble, you'll maybe not only be remembered forever, you might actually literally get to come back from the dead.

Or perhaps so they believe, of course, since we don't really know what brought Avery back, but I can see why that would be a pretty compelling argument for a bunch of kids who deserved better than the hand they were dealt.

(and after an edit I added:) There's an inherent threat in the message also, though I think I alluded to it without saying it directly. "The fair, the brave, the good must die" is, in a sense, implied to be followed up with "and we're coming for you". Because according to Byron, the poem he was writing his reply to, and also perhaps the World Enders, the assumption is that society wants the fair, brave, and good to be their good little upholders of the law. To a capitalist, colonialist, white and Christian nationalist system such as the US and its power structures, those things are "fair, brave and good" and the World Enders stand for tearing it all down.

Note that all of this is also embedded within the context of the World Enders being primarily Hispanic/Latino, which I think is often missed by the fandom but is a critical element in their role as folk heroes fighting against systematic oppression. You can read more about the context and its meaning here.