Kahler-Jex

Kahler-Jex was a doctor of the Kahlers.

He sought nothing more than peace and only committed heinous deeds for that end, feeling endless regret for the rest of his life.

Biography
He performed hideous experiments on members of his race in order to transform them into cyborg war machines. One of them, Kahler-Tek, later known as the Gunslinger failed to be shut down once the war on his planet ended. The Gunslinger hunted down the people that experimented on him and Jex escaped to Mercy in the United States. As self-inflicted punishment, he decided to aid the people with electricity from his ship and cured the townfolks' cholera. He became good friends with the marshal.

Tek tracked Jex to Mercy and cut off supplies to the town. Several weeks later, the Eleventh Doctor, Amy and Rory Williams arrived. Finding Jex's ship, the Doctor discovered his crimes. He nearly handed Jex over to Tek, only to be talked down by Amy. Mercy's marshal Isaac was accidentally killed when he pushed Jex out of the path of Tek's weapon. In his dying breath, Isaac made the Doctor marshal. Distraught by what he'd done, Tek made a bluff: hand Jex over by noon the next day or the town would be destroyed. In a duel, the Doctor distracted Tek and Jex escaped to his ship. Jex, feeling guilt for the experiments he conducted, committed suicide by blowing up his ship. (TV: A Town Called Mercy)