When Sarah Immeinu was abducted by the Egyptians and Avimelech's thugs, the reason why she was passed as Avraham Avinu's sister is because they might kill him if they knew he was her husband.
Why did Avraham Avinu think that they would kill him? If anything they would steal Sarah Immeinu and that would be the end of the episode. What benefit would the bad guys get by killing him?
Let's assume that those bad guys did kill the husband to steal the wife. What benefit would they have by killing the husband? If anything, they would be piling sin on top of more sin by stealing a married woman AND killing the husband.
Radak says that in those days, the only legal way to sleep with somebody's wife was to kill her husband on some pretext. The people believed they were sinning every time they slept with her, but killing her husband was only a one-time sin, if that. After that, the woman was a widow, and so was permissible.