“Vigilante” is episode twelve of season one of Threshold.
Ramsey is in bed with a stripper he’s picked up when a man breaks into the room. The man has enormous strength and kills the stripper quite efficiently. It turns out that the man has been infected to some degree, but his infection isn’t like anyone else’s, and he isn’t fully alien. Instead, he’s sort of in-between.
When the stripper is tested it turns out that she is infected, and is fully alien – which causes Ramsey some worries, given what he was doing. So why is the man killing other infectees? The stripper was the fourth victim he’s killed; two of the others were full blown infectees, but the third simply showed elevated theta waves, showing exposure to the alien infection. Just like Molly, Cavennaugh and Lucas, who has recovered from the infection he caught in “Outbreak”, do.
The man is a medical doctor and he’s hunting down and killing all the aliens he can sense. Unfortunately, not everyone that he senses that has been exposed is an alien, which means innocents are also in danger. He does provide a lead to Captain Manning, as it seems he had been tracking him.