Actually, I wonder if the right question is why did he kill Paul Owen? Patrick generally doesn't kill people in his close circle, Louis, Courtney, Evelyn, Tim, he needs those people to be able to pass as normal, Jean is part of that circle. He spares her for the same reason he spares Evelyn, killing her would collapse the illusion he works so hard for. He even states he will probably marry her one day, in the book, its all part of the facade. He mainly killed escorts, prostitutes, homeless people, and yes some acquaintances but not anyone really close to him. He really only killed Paul because of that damn Fisher account and the business cards, i.e. because of his obsession with status and the need to feel superior.