OKAY SO THE THING I FORGOT FOR KARA IS THAT SHE'S SUPPOSED TO DIE A TRAGIC DEATH IN THE FUTURE SAVING EARTH FROM IT'S GREATEST THREAT. and she knows about this!! but the knowledge of it has been telepathically sealed away in her head by saturn girl, one of the legion of super-heroes members. the idea behind it was that when it was time the knowledge would be unsealed and she'd die with at least her love interest beside her BUT THEN THE REBOOT HAPPENED SO WE DON'T KNOW HOW/WHAT WOULD KILL HER.