Removing an EnderDragon from world impossible

Summoned an EnderDragon :
/summon EnderDragon ~ ~ ~
Then, when appliying this command :
/kill @e[type=EnderDragon]
It writes in tchat :
Killed Ender Dragon
But it's still here, I can hit him and everything
Whan teleporting the EnderDragon into the void
/tp @e[type=EnderDragon] ~ -1000000 ~
It doesn't lose any life. Seen cuz I teleported it back to me :
/tp @e[type=EnderDragon] @p