Recently BBC made public the profile of the Gunslinger. If you watched A town called Mercy you are already know him.
Tasked with ending a long and bloody war, Kahler-Jex and a team of scientists created an army of deadly cyborgs. They took â€˜volunteersâ€™ and told them theyâ€™d been selected for special training. But in reality these unwitting victims were experimented upon, their bodies fused with weaponry. And finally, they were programmed to kill. The cyborgs formed a devastating force that routed their enemy and ended the war in less than a week.
â€œWhen he starts killing your people you can use your justice!The Gunslinger
Following their victory, all the cyborgs were decommissionedâ€¦ All, that is, but one. The circuitry of the cyborg created using Kahler-Tek was damaged in battle. He went off-line and like Frankensteinâ€™s monster, began hunting down his makers. He tracked the last of them, Kahler-Jex, to Mercy but wanting to avoid unnecessary bloodshed, he would not enter the town and simply waited for his quarry.
The Gunslinger unintentionally shot Isaac but after the death of Kahler-Jex, the Doctor persuaded him to remain on Earth and he became Mercyâ€™s guardian angel. This creature of war eventually found a purpose in peace: to protect it.
See Making of Gunslinger.