Actually Vento, he ALLEGEDLY killed her NIECE, who was his wife. It's not established in the episode whether he actually killed her or not, but it's Abigails sucspicion or belief that he did that's the motivation for her murdering him.

Yes, you're right.You know if Columbo investigated the death of her niece, we would know exactly if he were murderer or not.

Yes, because we would have seen him do it!