Endless Night; by Agatha Christie.

A man has more than one chance, of doing well and taking the higher path, and is free to do so at every opportunity presented. Some, however, do not do so - hence the title.

Title taken from a well known poem - while there is murder and mystery all right, this time it is more about human faults and depravations than about a detective solving a murder, though solved it is - by a fortuitous accident rather than anyone intelligent applying mind to the clues. The solution is presented finally by the killer, with readers left wondering about their own prowess as riddle solvers.