Though I absolutely hate grunge, it wasn't what killed metal. Metal commercially killed itself because it got too complex for the average fan to grasp. In the late 80's/ early 90's you had all these great bands releasing monumental albums, that were just too technical and powerful to sell to the mass. Gargoyle "Nothing is Sacred", Sanctuary "Refuge denied", Wicked Maraya, and the list goes on and on. So the labels turned to grunge which was the complete opposite. Simplistic, mellowed out and with whining replacing metal's rage and energy. So that started selling, and when the big bands like metallica started borrowing elements from that sound, metal went to the underground. Of course painkiller came out in 91 and send them all straight to hell but that's another story alltogether
Leee is right about the plastic fashion show, at the end of its peak poser metal adopted the pop mentality and produced songs catered to sell, hence lost the "street feel" of bands like Skid Row and such. Grunge did have soul at it's origin, but it wasn't long before it turned into a dope show, the washed up icons of which you can see even today (courtney anyone?). All in all I find it to be a plague in the music industry, not because it hurt the sales for heavy metal, but because it basically killed technical and powerful music in the market.