Cold Blood

[MWB]

I just watched "Cold Blood" on telly yesterday with Jemma Redgrave & Matthew Kelly. It was very good but I didn't get the ending. Why did that guy kidnap David's sister But she said David would never harm her? Why did David draw a picture of the guy who shot his wife & her policeman lover? Is it because he is the one to get David's revenge for him on Eve? So confusing? Please someone elucidate me. Thanks.