You cannot kill them. If you´re on PC you could do it with console commands or mods. Otherwise they are immortal. if you're on PC and want to kill him in a fair battle use "setessential <BaseID> 0" in the console.

Can you kill the imperial legates?

Because you can't . The game don't allows kill VIP ppl , because you need them to do quests or ... Legates and Generals are essential. ... It's a nice game.

Can you kill Legate Fasendil?

Legate Fasendil is an Altmer Legate in the Imperial Legion. Fasendil is in charge of the Imperial soldiers at the Rift Imperial Camp. He is an essential character, which means he cannot be killed.