Can he die, after "what so ever" quest anything???

Never. He's also the most powerful NPC.

Essentials in Skyrim can never die no matter what you do. Nothing more annoying then going to clean out an Imperial camp when roleplaying and the legate being essential and never dying. I believe the same applies to Oblivion but I think maybe a few NPCs may lose their essential status due to quest developments in that one

Even if he could he would kill you anyway along with the other 3.