The Blind Side — Patricia Wentworth

Craddock House was so full of squabblers, gossips,.-feuding family, and eavesdroppers that Ross Craddock's killer must have felt quite at home. There was Lee Fenton, Ross's first cousin, heard calling Ross "a swine" and subsequently sleepwalking (so she claimed) into the victim's flat. There was the lovable spinster aunt whom Ross was cruelly evicting and who just happened to enter the house at 2:00 A.M.--in time to see a figure fleeing. And there was the beautiful blood-smudged Mavis Grey, quite capable of looking after herself; and Bobby Foster, Mavis's enraged suitor, who managed to leave his fingerprints all through the house.

Indeed, with so many clues and suspects throwing themselves at Inspector Lamb, how could he possibly notice the deadliest one of all?