Krall was a long-lived former Starfleet captain who attacked the Enterprise and Starbase Yorktown in the Kelvin timeline, driven by hatred of the Federation.
Krall first appeared as an alien warlord on the planet Altamid in the Kelvin timeline. In Star Trek Beyond, he lured the Enterprise into a nebula with a false distress call and destroyed it with a swarm of small ships. He was searching for an ancient bioweapon and captured most of the crew to drain their life force.
Kirk discovered that Krall was really Balthazar Edison, a human soldier who had become captain of the USS Franklin. Stranded on Altamid when the Franklin was lost, Edison and his surviving crew used alien technology to extend their lives by draining others, which also changed their appearance. Bitter that the Federation had made peace with old enemies and believing it had abandoned him, Edison set out to destroy it. He attacked Starbase Yorktown with the bioweapon, but Kirk stopped him, and Krall was killed when he was blown into space with the weapon.
Krall served as a critique of the Federation's ideals from someone who felt left behind by them.