There are 2 basic types of undead:
Mindless, Those long dead after being ressurected and who barely have a soul or a brain, they can only only take orders and understand basic combat, aka ghouls, abominations, zombies etc

Mindful, they were ressed back right after being killed, they have will power but most are still under the control of the lich king while those who freeded themselves are the forsaken or ebon blade.

Now the Forsaken is ruled by a number of mindful undead, like syllvannas etc but they can still summon dead of there own, most of which are mindless

Now the Scourge have a lot of mindful of undead but they are still under the control of the lich king, while some others woolfully are ruled by the lich king, like cult of the dammed members who have become undead, Anuberians seem to be mind controlled as they were full force attacking the scourge back during the spider wars(either that or they will follow there king, Anub nerak to the death).

After the lich king is killed I am sure the Mindless Scourge who are under the control of the lich king will go insane and attack anything in sights, anything they see they will attack, no matter if a fellow undead, a human, orc etc