John Barnett

John Barnett is the central antagonist of the X-Files episode "Young At Heart", alongside Joe Ridley - he is one of the show's many "monster of the week" villains who explore paranormal events and phenomena outside the series official "mythos", which tends to involve alien conspiracy : Barnett's case involved the concepts of immortality and age-reversing, as well as a recurring theme of criminals from the agents pasts returning to haunt them in the present.

History
John Barnett was a notorious bank-robber and criminal with homicidal tendencies, so ruthless that he would even take his own partner as hostage - he would be one of the first true villains Agent Mulder ever faced after joining the FBI and was eventually secured and arrested.

Barnett officially "died" in prison of a heart-attack in 1989 but had actually been the test-subject of Joe Ridley's experiments on reversing the ageing process - thus he went undercover for several years and planned revenge on Mulder.

In 1994 Barnett returned and began stalking the agents, ultimately firing on Scully (who survived due to wearing a bullet-proof vest) - in the subsequent confrontation Barnett would be shot dead by Mulder.

Abilities
due to Ridley's experimentation Barnett has not physically aged at all, despite several years passing, this means that he is biologically immortal - though he is still as vulnerable to death by violence as any other man : as is shown by being shot to death.

as a ruthless criminal he also has skills in firearms and is fully capable of murder without a second thought.