This is another thing I bring up, but along with depositing that money back the next day, maybe Ken makes another mistake with Miss La Sanka.
She's at least as excited by him as the money, so he could string her along for who knows how long. Even if EVENTUALLY he might get nervous enough to murder her.
This is also true: La Sanka was a non-threatening blackmailer, Frankling took a huge risk killing her for just 15k dollars, which by his own admission were a small amount for him, with Columbo already on the case.