What has been seen is that the Scourge are susceptible to being controlled by demonic elements as well, as was seen with the Dreadlords in the Plaguelands as the Lich King was weakening and recalled Arthas and what forces he was able to retain control over, what hadn't regained its free will and sided with Sylvannas came under the control of Varmiathras and his brothers. Isn't there also a quest that hints at the possibility of Forsaken being able to recall atrocities committed while they were enslaved as lowly ghouls and zombies amongst the Scourge? Anyways, in the absence of the Lich King's Control and sustained free will, scourge simply fell under the sway of Varimathras, Detheroc and Balnazzar... the possibility of a third or fourth state of undead exists beyond "Enslaved" and "Free-Willed," these being "Freed and insane" driven mad by the trauma of experiencing death, undeath and all the things they may remember having done while enslaved... the erratic behavior could be exceptionally prone to violence.

Then they could possibly be a "Freed and Instinctive" state, possibly lacking a proper soul, the body is in a state of unliving without the benefit of a fully functioning mind/soul to guide it and goes into an instinctive mode that includes "Feed" which turns the horde of undead into something more akin to locusts. It is entirely possible that some of these carry an adequately potent form of the Plague of Undeath that victims themselves eventually rise again, very possibly as one of the two states of mind previously mentioned, or basically your stereotypical Zombie outbreak. It is entirely possible for Demonic elements to attempt to exercise control over these ravenous bands of undead as well... and there are plenty of Scourge and Demons spread amongst both continents to make this a real possibility, and 3 Dread Lords were able to retain control over large numbers of Scourge even while the Lich King was around (Albeit weakened).

It has been mentioned several times before throughout WoW.com that overall, the Alliance, Horde, and the Crusade have not been faring well against the Scourge overall, and that the Cult of the Damned have infiltrated extensive portions of both the Alliance and the Horde, so knowledge of Necromancy and possibly the plague itself (especially amongst RAS survivors) will very likely survive the Lich King's fall with these cultists that had not yet been rooted out. Indeed, lore wise the original campaign to "Wipe out the Scourge" has devolved into an attempt to cut off the Serpent's Head due to disastrous events at Wrathgate and elsewhere as the losses mount. The Knights of the Ebon Blade which are providing the greater portion of success have done so primarily by taking out local leaders of the Scourge that provided finer tuned direction rather than wiping out everything in sight. This suggests the the Undead are less a normal Serpent and more resembling of a Hydra with only the Lich King as a proper head at present, but upon cutting it off, without putting a new head in place immediately, dozens if not hundreds of heads spread across the face of Azeroth would grow in its place, free of whatever remaining conscience remained of Arthas that had previously held them all back.