They do respawn. You have to beat the elite four again to do so, though. If you do catch them, they don't respawn. Hope I helped!
EDIT: If you happen to kill the legendary after you beat the elite four again, you have to beat them ANOTHER time. The legendary will keep respawning until you catch it and you've recenty beaten the elite four.
>if I was to kill them again, would I beat them again?
commented 15 minutes ago by DEOXYS123
