Kahler-Tek, also known as ‘The Gunslinger’ was a Kahler cyborg, one of many humanoids who was experimented on by Kahler-Jex on his home planet of Kahler. Jex wanted to create a super soldier race so he could win the war his people were engaged in.
Tek and several others were converted into powerful half men, half machines: A terrifying army of armed war machines featuring powerful energy weapons and built in targeting computers. Kahler-Tek became damaged on the battlefield and slowly his original personality returned to him. He became aware of what he had become and what Kahler-Jex had done to him. Enraged, he set out to track down Jex and his scientists who had turned him into a killer cyborg, hell-bent on revenge.
Years later, the vengeful Tek tracked down Kahler-Jex to Earth, where he had taken on the role of the Doctor in a town called Mercy. Tek lays siege to the townspeople, and when the Eleventh Doctor, Amy and Rory arrive they face an ultimatum – Hand over Kahler-Jex to face his fate or the gunslinger would kill every innocent civilian in town, despite his strong instincts not to harm innocents. When Jex expresses genuine regret for his past crimes, he chooses to commit suicide rather than face his cyborg creation.
Kahler-Tek believed he no longer served a purpose: All those who had mistreated him had died, so he decided to self-destruct in the desert. However, the Doctor persuaded him to stay in town to take on the role of protector of the people and keep them all safe from outsiders.