He can corrupt Azeroth, sure. Just kill us and be done with it. He also needs the Soul of Argus, you know that yes? Also, why would he not know of our invasion of Antorus? The fuck? He's right there, and he clearly knew of our ritual with Argus and the Titan spirits, which is why he awakened Argus to begin with. Why would he not realize we were invading Antorus and stopping his ultimate Dark Pantheon plan from unfolding? The fuck???
Zovaal at least assumes us as literal fodder and knew Sylvanas and co could hold us off long enough so he could obtain the last sigil. Hell, he DOES actively try to stop us a couple of times also, granted he doesn't kill us cause of his "serve all" philosophy, which is also dumb asf, but that in itself doesn't matter with my point.
Sargeras wants us DEAD! Why not fucking KILL US??? WE ARE RIGHT THERE!!!!