Why was Eileen Mace killed in the novel, The Whistler?

Eileen Mace, Junior Mace’s wife, was shot by Delgado and put in bed with Son to make it appear they were having an affair. Her death was a ploy by Dubose to get rid of Son and Junior, both of whom were against the building of the casino. Junior was framed for her murder, found guilty and sentenced to death.