Some killers have moments when they sound so sincere that even the viewer is tempted to believe them, even though they've already seen the killing. This is mainly due to the actors.

There are two that occur to me right away.

Tommy Brown
When Columbo says he's from homicide, Tommy of course says "Homicide?!"
But he says it exactly the way you'd expect an innocent person to say it.

Adrian Carsini
The moment I mean is when he tells Columbo to do what he wants with the check to Ric, and he says in that sad little voice, "It WAS meant as a wedding present, no matter how things turned out."

Johnny Cash and Donald Pleasence each make you really want to believe their characters during those two moments.

What are some others?

Love the Jonny cash one!