He ordered Stiles to be sedated when they put him in solitary and happily tells Sheriff Stilinski that they sedated Meredith, too.
Brunski was a bully in school and used to bully Coach Finstock. When Brunski comes to the high school looking for Meredith, Finstock uses the opportunity to get back at his tormentor and uses Brunski's own taser on him.
When Parrish, Lydia, and Stiles wanted to see Meredith, Brunski got in their way, only relenting when Parrish threatened him over a drunk driving charge. He harassed Stiles about the overdue bills, as well. When Lydia discovers the names on her grandmother's message, they go to Brunski to try to bribe him into showing them the medical records. He pretends to go along with it but instead captures Lydia and Stiles, because Lydia is on the deadpool. Brunski admits to having killed all the people on Lorraine's list, claiming that he was saving them and setting them free. He has Lydia listent to a tape of her grandmother dying in which she begs him not to he hurt "Ariel," Lorraine's nickname for Lydia.
Brunski plans to kill both Lydia and Stiles with drugs, but Parrish shows up and shoots him instead. As he's dying, they accuse Brunski of being the Benefactor, but he laughs and claims that it isn't him. Instead, he was being controlled by Meredith, who reveals herself as very much alive.