I don't understand why killing Winterson was Max's biggest regret. He knows she was involved with Vlad and his plan and also that she was going to kill him and Mona.
I get that Max and Winterson were two sides of the same coin, two seemingly good cops that became romantically involved with criminals and killers. I just don't understand why Max feels that much regret over Winterson when Mona had his back to the end. Also why he is building her up to be a better cop than she was especially since she went out with the intent to commit murder.