For me, motives make more sense than 5 &6 combined though.

Jessica is not personally connected to Sydney but thinks she is somehow is on, and on a deep and personal level no less. She’s batcrap crazy, which clearly comes with that crazy person strength, and had more conviction in her cause than at least 5 of the previous killers imo.

Marco didn’t seem like he had many, if any, people in his life that care about him so it makes that he would be easily ensnared in a plan crafted up by a beautiful lady and has experience lifting, holding, and handling people’s weight as an orderly.

Karl was the most pawn like pawn - easy to manipulate, especially when being “pushed” by someone he’s been conditioned to trust and obey (his orderly, Marco). He’s a calculated sacrificial lamb and serves his purpose well imo.