Who Pays the Piper? by Patricia Wentworth

I always get what I want.' Thus spoke Lucas Dale, a wealthy man in his early forties who had come to live at King's Bourne, former family home of Susan Lennox.

He meant to have Susan as his wife, but she was already engaged. This did not deter him in his purpose and he resorted to blackmail to obtain Susan's promise to marry him.

He is murdered and suspicion falls on Susan's fiance who was heard to threaten his life.