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Frank Nollette is the true antagonist of the X-Files episode, Roland - despite not being revealed as such until later in the episode (as is common in many media, especially mystery dramas such as the X-Files).
Nollette was a brilliant scientist working on a means to make a powerful engine surpassing Mach 15 alongside several other scientists, one of whom was Arthur Grable, who died a few months into the project.
When many of Arthur's former scientists began to die in horrific accidents it became apparent Arthur was using his twin brother (Roland) to try and finish the research - though the agents were puzzled as to why Arthur would be killing men he used to work alongside.
This was soon revealed by Nollette however, as he gloated to Roland that he was stealing Arthur's research as his own and even went as far as trying to kill Roland after he sabotaged Arthur's cryogenic tank - however Arthur was able to keep control over Roland and knocked Nollette out before locking him in the wind tunnel.
Arthur (still possessing Roland) began to turn on the engine with the intent of murdering Nollette but Mulder and Scully managed to convince Roland to save him and "let go" of Arthur, whose brain apparently perished as the cryogenic tank finally ceased to function.
Due to Roland being severely mentally handicapped it is unknown if Nollette's betrayal of Arthur was ever exposed and due to the paranormal nature of the case it is unlikely he would be convicted: however in a sense he did receive some punishment in the fact he was almost killed (he may also of been arrested by Mulder and Scully if they found evidence of him carrying a firearm or his involvement in sabotaging Arthur's cryogenic tank).