The last straw
Because the police lacked any evidence to support their claim that David was guilty, they were unable to take any serious action. As a result of the tragedy that occurred when Martha went missing, her parents were unable to recover their relationship and ultimately ended up divorcing.
They had been at one another’s throats very often, but in the end, they were able to put their disagreements behind them and maintain the family unit together. The disappearance of Martha served as the last straw for them.