This isn’t the first time Fuches has worn a wire. Back in season 2, he was arrested in a hit gone wrong. Janice’s partner, Detective John Loach (John Pirruccello), offered to keep him free if he entrapped Barry. In the season’s fourth episode, “what?”, Fuches had second thoughts but an unknowing Barry still spilled secrets. They only escaped because Loach tries to blackmail Barry instead of arresting him (see the next episode, “ronny/lily” for how that turns out).
Fuches’ betrayal stung and Barry again cut him out. So, Fuches tracked down Janice’s body. Posing as the private investigator “Kenneth Goulet,” Fuches brought Cousineau out to the body. He intended to stage a murder-suicide; call the police with Cousineau’s phone and “confess” to killing Janice, then kill Cousineau in front of the body. But when the time came to shoot Cousineau, Fuches hesitated. With the police moments away, he told Gene the truth about Barry instead, then ran.
Understanding why Fuches tried to take his revenge this way will contextualize his actions in “yikes.” Over the course of season 2, Barry began to see Cousineau as a father figure. In “what?”, he opened up to him about the incident that got him discharged (he killed a civilian in Korengal). Cousineau, though stunned, didn’t condemn Barry and told him he believed in his better self. Cousineau would later talk about Barry to a seething “Goulet,” claiming he was the one who turned Barry’s life around. Fuches was so jealous of Cousineau’s relationship with Barry that he decided to destroy it — one way or another. Barry disavowing Cousineau and saying he should’ve listened to Fuches must’ve hit the old bastard right in the heart.