1) So if the Void Lords corrupt and destroy life....and Sargeras corrupts and destroys life....where is he any different. Oh the ends? See next question
2) If Sargeras is to kill everything and remake it, remake it how? And in what way does he "Void-Proof" all of creation?
3) How does killing everything first change anything with the Void Lords? They aren't a part of the prime material plane. If Sargeras were to succeed, wouldn't the only things left be him....and the burning legion? The remnants of all reality is demons, their lord....and the representation of pure darkness? Who wins that fight? I'm go out on a limb and say it's a lose-lose.
Bonus question-If say we completely destroy the legion, aren't we the baddest beings in the entire universe? Does that even make sense?
*Burning legion=infinite immortal beings that can only die if killed in a placed soaking in fel/the twisting nether
*Denizens of Azeroth=Finite beings that can be killed quite easily (in comparison) and never return. Easily corrupted, easily seduced. Come to blows with their own kind, and can't even see past species differences, even when they are being beat over the fucking head with demon invasions.
We're doomed.