Posting this knowing something similar has probably been discussed. But I can think of four victims who, for some reason, felt it was good business to either blackmail or pretty much tell their killer that they were going to expose their deeds: Edna Brown (Swan Song) leaving Tommy Brown working for peanuts while hanging the threat of exposing his statutory rape over his head; Jesse Jerome (Now You See Him)...again, blackmailing Santini about his Nazi past, while squeezing him out of most of his earnings; Bertie Hastings (Bye Bye Sky High...), after giggling like a baby when he was tickled, telling Oliver Brandt how he was going to expose his embezzling ways; Gene Stafford (An Exercise in Fatality), letting Milo know "you're a thief" and that he had enough evidence to compile a lawsuit against him. Feel free to add another victim who probably could've survived had they simply handled things differently :)

In "Death Lends a Hand", although Brimmel tries to blackmail Mrs. Kinnicut, she turns the ole' switch-a-roo on him and threatens to expose him to her husband.

Picked a bad spot to do it.

I like this one, Quid. Totally forgot about it, but yeah, I remember watching it thinking, "Leave Brimmer's place before telling Brimmer that you're going to tell your husband about him!" I love the way she stole his leverage. Just wish she executed her exit better.

I would say Viveca Scott is antagonized pretty heavily by Karl Lessing before he gets his lights put out.

Vanity is one of Viveca's traits (not that there's anything wrong with that), so one of Karl's biggest mistakes is making her think that SHE'S one of his blackmail demands, then letting her know it was a joke!

In MURDER, SMOKE AND SHADOWS, Leonard Fischer goes on a pretty good rant towards Alex Brady making all sorts of threats.

There were also a couple of paramours "sticking it to the wrong guy."

Troubled waters and Etude in Black come to mind.

Harry Stone is pretty rough when he orders Nelson Hayward to give up Linda. Maybe it makes perfect sense for Nelson's career, but he's very crude about it.
(Plus, he accidentally gives Nelson the plan for the murder, because he's the one who invented those death threats.)

Varity Chandler threatened not only to expose Prince in Ashes to Ashes but also antagonized him in the funeral parlor to no end. Telling him that it was not blackmail she was after, it was to ruin his career. She had planned a big tv spot with his picture in the background to expose him thereby ruining his future.

In the end she got burned. Literally. But then again so did Prince thanks to Columbo

There's at least one other case of the victim making the mistake of NOT being a blackmailer. I know I've said it nearly a dozen times, but Lenore Kennecut announces to Brimmer that she's going to expose him to her husband as a blackmailer after confessing her affair, when it would be a lot better to use "counter-blackmail," since each of them can cause trouble for the other. Or promise NOT to cause trouble for the other.

Have to agree, Verdammt. I probably didn't think enough about the ABC series, but yeah, "poor, Lenny"....all of that anger, telling Alex how he's going to ruin him...and then suckering into Alex' ploy to get him to hold off until he shows him the film was a fake.