One of the most dramatic decision-to-kill scenes is in "Make me a perfect murder". We knew Kay was ******* he had told her about his plans to leave without her much earlier in the fairly long scene. But the moment when she actually made the decision was when he gave her the car keys. At that point her "murder music" comes on, and as she is hugging him goodbye, she fixes her gaze on the gun he had thrown on the bed.
A funny one is in "Double Exposure", after the blackmailer Roger leaves Dr. Keppel's office, Keppel gives an annoyed sigh, as if to say "now I have to kill him too."