Detailed Explanation
Love Quinn, Joe's Season 2 love interest turned wife, is revealed to be just as deadly as Joe himself. Her first kill was her au pair Sofia, whom she murdered as a teenager. She killed her first husband James Kennedy when he tried to leave her. In Season 2, she kills Delilah Alves (throat slit) when she gets too close to exposing Joe, and her own brother Forty dies partly due to her actions. Season 3 reveals her full nature: she murders Natalie Engler (axe to the head) for flirting with Joe, kills Gil Davis (makes it look like suicide), and finally attempts to kill Joe himself with aconite poison before Joe turns the tables and kills her first.