Although of course it's very fun to look at this through the lens of the formula-definer for Columbo, complete with a "just one more thing" moment, what struck me more was the emotional dynamics. I find Hitchcock's ability to express nuanced emotion beyond those that are necessary for the genre hit and miss, but this is definitely a hit.
I often joke about plots that could be solved with polyamory, but this one stands out as a particularly tragic example. From the first scene, it's so clear that Margot truly loves both Tony and (for some reason) Mark. Not only does she want them both in her life, she does her very best to get them to enjoy one another's company as much as she does. She is fully ready for this to be a ménage à trois, she just can't quite find the words to make it happen. And that's what really makes Tony so effective as a villain: not just his callous willingness to murder his wife, but his absolute inability to understand her.