actually u never see Illidan killin Magtheridon, he walks up to him in the cinematic and says his reign is over, shortly after tht Kil'Jaeden shows up, but i agree Illidan DID get cut by Frostmourne, but then again he is half demon and fairly powerful, personaly i think Blizzard made a HUGE mistake when they made key characthers killable in WoW, i mean....are we ever gonna see a new strategy game? and if it DOES show up, wht is it gonna be about? by tht time u prolly made it to Northrend and killed Arthas....which would suck big time if u ask me
you only truely kill a boss in wow when you can loot the head, like Head of Onyxia, MAgtheridons head,
Head of Ossirian the Unscarred, heart of hakkar etc.
we dont kill illdan, we just defeart him(for now.) well thats what the wow lore freaks in my guild says anyways. :)