Blade cut through flesh, blood, and bone as limbs were severed from the body in an instant. Zane's Sickels tore through the undead in front of him at a speed unrecognizable by a standard human eye. He himself was covered with the coagulated blood of the monsters which he slayed.
Could he have peacefully walked from one end of the city to his destination? Yes. Was he going to give up a perfect opportunity to farm the undead for their blood crystals and resources? Absolutely not.
Of course, there was only one problem with cutting through thousands of monsters that lie between you and your destination. You were basically creating a giant path of blood and body parts leading directly to where you were headed.
Because of this, after cutting through a few hundred undead, Zane ceased his actions. To kill that many undead, in that short of a time span. You would either need to mobilize a large force of awakened survivors. Or you would need a Horde King to have done it.