Arnie MacLaren is a former NYPD police commissioner and disgraced candidate for Director of domestic Security. MacLaren was responsible for the murders of Samantha Savage, a porn star he had an affair with, and Helen DeVries, Samantha's editor who was also sleeping with him.
When he was Milligan's C.O., he violated ethical standards by helping to cover up Pete's drinking on the job, when he assaulted his wife and son, and his son's drunk driving.
Along the way, while he was ex-commissioner, the President nominated him for Director of Domestic Security, and he had affairs with Savage and DeVries. When Savage sent a selfie with him sleeping half naked next to her to DeVries, she was furious and confronted Maclaren in his extra apartment. Maclaren went to Samantha's apartment to get her phone and the photo, desperate to keep his affair a secret and to get rid of any evidence of it. He was married, his wife was expecting, and he was soon facing Senate confirmation, so he couldn't risk anyone finding out about his infidelity. Plus, there was always the risk of Savage blackmailing him with the photo once he was Director of Domestic Security. In Savage apartment, they ended up fighting, Savage went for her gun, he overpowered her and killed her with it.
Later when DeVries became a suspect in Savage's murder, he had Milligan frame her for Savage's murder by planting evidence and producing a false witness. Giving Milligan a shirt with Savage's blood to plant and the gun he used and took from Savage apartment; all to frame Helen. Which got her arrested and charged for the murder.
MaClaren later withdrew his candidacy when his affair with Helen was discovered. And after she was out on bail, Maclaren didn't want to risk her talking about their affair, and what happened with Savage. So, he threatened Milligan into killing her. Milligan complied and shot her to death in her parking garage.
Milligan eventually cooperated and told the truth and everything about Maclaren and testified in court.
MacLaren was found guilty of murder and imprisoned, resulting in his powerful friends severing ties with him. (L&O: "Publish and Perish")