We killed Murozond a while ago at this point in what is probably my favorite 5-man boss fight of all time - though of course now that there are Cataclysm Timewalker dungeons, you might have done so more recently.
But the thing about time-travelers is that just because you've killed them doesn't mean that they won't trouble you anymore. Even if we did truly end his life in the End Time, and even before we get to the whole "did preventing the Hour of Twilight, and thus the End Time, from happening mean that we also retroactively prevented ourselves from killing Murozond" paradox, there's still the plain fact that we have no idea how long Murozond lived after he stopped being Nozdormu in which he probably wrought a lot of chaos along the timeways.
And even if we set aside the fact that a slightly younger Murozond could literally show up at any moment and unleash catastrophe upon us (hell, there's nothing that says he hadn't already killed future versions of us by the time he went to hang out in the End Time,) there's also our very own Nozdormu who seems inevitably fated to undergo the same corruption.
The story goes that Murozond was corrupted, like Deathwing, by the Old Gods. But that means that even though Nozdormu knows exactly what is coming from him, there's nothing he can do to prevent it.
Of course, part of the tricky part of writing time travel stories is figuring out just how much can change and how much can't. It could be that if Nozdormu were to try locking himself in unbreakable chains or even trying to off himself before he gets corrupted, this very act might be the thing that turns him into the monster he fears. But of course, doing nothing would allow external events to lead him down that path.
The mere fact that the Infinite Dragonflight has ever existed means that we could potentially never truly defeat them unless we could account for every last Infinite Dragon's own personal timeline, which is probably going to be very difficult given that the Infinites are all former Bronze dragons and thus the very people we'd need to help track them would be inadvertently giving their future, evil selves all that information as well, allowing them to keep one step ahead of the game.
But there's a kind of weird hitch here.
After the Hour of Twilight, all the dragonflights lost their power over their old domains. A Bronze Dragon is now not really much different than a Red Dragon, except one breathes fire and the other breathes sand (not respectively.)
Yet Murozond seems to possess the powers that Nozdormu had when he was an Aspect.
I think the most obvious explanation for this would be that Murozond, in his corruption, is either empowered by the Old Gods or finds some way to restore himself through some Titan tech or some form of more mundane time travel (if you can call any time travel mundane) to return to the time he was created and siphon off some of that power (maybe it goes wrong, which explains why the Infinites look so freaky.)
But here's something we hadn't really considered: What if Murozond isn't actually Nozdormu?
It's been Nozdormu's narrative for his entire life as an Aspect that Aman'thul gave him a vision of his own death to keep his ego in check. But in the Dawn of the Aspects, it seems that the Aspects didn't actually have a sit-down talk where the Titans layed everything out for them in explicit terms. The Titans probably weren't even on Azeroth at the time, but empowered them from afar.
Was that vision really a warning to "remember thou art mortal?" Or was Aman'thul telling Nozdormu who his biggest adversary was?
There was some communication there - Nozdormu was told to protect the one true timeline. But Murozond died in a different timeline - a future that never came to pass. We assume he traveled there from ours, but that may not be the case.
We've spent this past expansion in an alternate universe that has a lot of familiar faces. What if Murozond is Nozdormu's doppelganger, and not his future self?
If that's the case, then the entire Bronze dragonflight has been preparing for the wrong sort of conflict.