Tret

Tret was a minor antagonist in Star Trek Enterprise, though his intentions were ultimate what some would consider "good" his methods were the direct opposite - due to an alien virus in the system he had come to a drastic course of action that consisted of killing any and all beings infected in order to prevent the spread of the disease, this would put the alien commander in conflict with the crew of the Enterprise when several of their own were infected by the virus.

History
Tret was an alien commander whose people had suffered greatly from a viral epidemic caused by a mutagenic virus originating from the Loque'eque homeworld. He enforced his people's quarantine of the Loque'eque homeworld by exterminating any alien who landed on the planet. He had at least one starship and incineration patrols at his disposal.

In 2153, Tret placed the Enterprise NX-01 under quarantine when his ship detected that one of Enterprise's crewmembers, Malcolm Reed, was infected with the Loque'eque virus. He demanded that he be allowed to board the Enterprise to eradicate the "infected organism", but Trip Tucker persuaded him to come aboard peacefully to discuss the situation.

In Enterprise's sickbay, Tret explained the virus and its origins to the crew. He then returned to his ship to deal with three more Enterprise crewmembers on the surface of the Loque'eque homeworld who had become infected during an away mission. Tret later attacked Enterprise when those infected crewmembers were returned to the ship, determined on preventing them from leaving the system. Tret ultimately relented when Doctor Phlox convinced him that he had successfully devised a cure for the virus.