Karl Hoffman was a serial killer and member of The Blacklist.
He was physically abused by his mother as a child. This abuse inspired him to become a serial killer. His targets were people who abused close family members. Finding victims through his jobs as a locum nurse (i.e. filling in at clinics that were understaffed), he abducted them and took them to a home-made operating room at his childhood home, where he would use different instruments to inflict all of the exact same injuries on his victims that they had inflicted on their loved ones, the compounding injuries resulting in their deaths. He was named "The Good Samaritan" by the media because he left the victims alive but badly wounded and called 9-1-1 to tell their location, giving the paramedics a chance to save them (though none of them is mentioned to have survived). He was initially pursued by Elizabeth Keen and her team. He stopped attacking people for a period of time for an unknown reason.
At the start of the episode, he gives Karen Brodine a drugged drink at a children's play area.
When she awakes from her drug induced sleep, she is tied to a operating table, where Karl proceeds to inflict various injuries upon her. When he has finished, he leaves her in her own vehicle and calls 9-1-1 as he walks away.
Later at work as an Agency Nurse in an emergency room he treats Melissa Wilkinson for and injury that he identifies as being caused by her abusive husband George. While treating her he reviews her medical history. After his shift is over he attends an abusers support group where he shares his his mother abused him as child, and how they have, after a long time, reached an understanding. After the sharing session he befriends George.
Later at home, he finishes brushing his wheelchair bound mother's hair, then he wheels her out to the garage where George is secured to the operating table. He explains to George that he is going to inflict the same injuries on him, that he inflicted on Melissa. He further explains that his mother is going to watch as this reminds her of the injuries she inflicted upon him, injuries she says were given in love, but he believes were to give her pleasure.
When the FBI interrupt him he makes one final attempt to kill his mother, but is shot dead by Elizabeth Keen.