A little hard to follow the plotting and action sometimes, because of Hammett's heavy gangster 1920s vernacular, and the level of deviance and betrayal by the criminals and police. But still a primal source for much of the crime fiction I love.
“It's an expression he used to describe what happens to somebody psychologically once they've committed murder,” Joel Coen told Time Out. “They go 'blood simple' in the slang sense of 'simple,' meaning crazy."