I think you guys are having memory leaks.
Certain mobs were set to run whenever they felt their life was in danger. Whether that be 50% health, 20% health or 1% health. If the mob was snared, it would turn to flee, but would do so slowly until it got to a certain percentage (perhaps 10%) then would stand there, snared. You are correct that a rooted mob would turn and fight - as an ex-Wizard, it used to piss me off when someone would root my snared mob trying to help, thus letting it hit me. Undead, afaik, never fled.
Take Drolvargs in Dreadlands and KC, definitely would try strolling away even if snared until they got to a percentage of health that caused them to stop in their tracks. Granted, this was some 6 years ago, so things might have changed. But this is how I made my career in EQLive as a wizard, so I am fairly confident I am not wrong about this.
Lastly, on the subject of "early runners" - you had SoW'd mobs that could run away at 50% and gather tons of their buddies to come get you (Nurga/Droga/Guk, anyone?). You had lower level mobs that you could easily beat (Orc Pawns) that would take off in a random direction after getting to 50% and you'd have to chase them down. I am pretty sure some of the Luclin+ mobs did not behave the same - so maybe my memories are of the old world (when life was good).
This is what I remember. If I'm wrong, I'm wrong. Shrug.