Are you serious with what he has done? read the books and lore around him? not only what he has done he have wasted sargeras time over and over and over again, when he could focused on somthing else.
Just read the books and look up his history, will take you prolly weeks of reading but, after you have done it in detail you will not argue against it.
Most ppl have just read the "hot" or mainstream story things about him, turns out... the least known facts are just as important, especially since blizzard never told what happened or how burning legion/sargeras acted becuse of him, but one thing is certain it has effected a lot.