In the Mid-Season finale of Arrow, Prometheus hurts Curtis Hall. Why doesn't he kill him? If he wants to get revenge on Oliver, wouldn't killing Curtis be really effective.
Because Prometheus doesn't want a straight revenge on Green Arrow by killing his teammate but wants to torment him. His motive is more to break Arrow and he is achieving it by breaking his team, Quentin Lance is disturbed mentally, Artemis is working for him, he knowingly/unknowingly created issues in Curtis's marriage, he made Arrow kill Felicity's boyfriend, Spartan got arrested back (not sure of Prometheus's involvement yet).
So his pattern of torment is not by killing Arrow's friends/family but to effect them mentally. So he is quite successful in getting his revenge but by non conventional methods.