Ruzek's loyalty was questioned on Chicago PD Season 5 Episode 7.
Denny Woods blackmailed Ruzek by threatening to press charges against his sister following her DUI arrest unless he planted incriminating evidence on Voight's computer.
Ruzek was about to follow through with the order but bailed at the last second, knowing he couldn't do that to his work family.
He still had to give him something worthy enough to open up a "misconduct investigation," so he handed over a video of Halstead punching a civilian in a bar while undercover.
The violent exchange happened while Halstead was trying to buddy up with Luis, an ex-ranger who was working with a crew that was kidnapping innocent children in exchange for ransom money.
The kid, Cody Barton, was taken while out with his nanny, who the ex-military gang shot dead.
His father, Ben, didn't have enough money to provide the requested $1 million dollar ransom so Voight stepped in to help out.
Halstead cozied up to both Luis and his hot sister Claudia, who worked at the bar, where the fight happened.
He was a persuasive undercover not only because of his dimples but because he shared the same military experiences as Luis.
Luis was hesitant to trust Halstead, especially when he kept asking about getting a job with his private security firm.
Finally, he was able to strike a deal; Luis would take get him a gig in exchange for some oxycodone, which Voight illegally provided Halstead.
When one of Josh's men went missing — thanks to Atwater and Ruzek — Halstead was unwillingly added to the "transaction."
Under Voight's guidance, Ben told Josh that he would not be meeting him with the money and that he'd called police instead.
As Josh was flipped out, Halstead persuaded Luis into doing the right thing. They weren't able to save all the children when they were serving but they had a chance to save this kid.
When they arrived at the location where Cody was being held, Luis and Halstead teamed up to take him out.
Then, they infiltrated the warehouse with Intelligence following suit.
Luis realized that Halstead, who went by the undercover name Ryan, had played him but it was too late because he was shot and killed.
Halstead was in a haze when he saw Luis dead.
He went to Claudia's house to fess up, but when he saw how distraught she was, he lied and didn't come clean about who he really was.