Leslie Williams, I can only agree with partially. She has one of the more ruthless and coldblooded set-ups for murder but it's only because the daughter character I think doesn't come off as particularly sympathetic that it ends up inadvertently making the viewer more inclined to side with Leslie.

"Uneasy Lies the Crown" was a more specific case of where I ended up feeling a bit sorry for the killer because his wife was a two-timer and seeing her go all hysterical over the death of her lover inadvertently sets up a "served her right!" dynamic.

I personnally like very much the one in the episode 43 "make me a perfect murder", played by Trish Van Devere.
She represents a very "human" killer, with a lot of feelings, who tries to play "though", especially in a man's world and profession especially at that time, but seems to have too many emotions, and be too compassionate to achieve her goals.
We can feel it even through the way the character is played, with a low voice, almost like a mother talking to her child.
Well it's the impression I had, anyway, and each time I watch this episode, I feel touched by this character, despite the cold-blood murder...