Getting back to the book (as opposed to films and radio series) .....
One thing I always found hard to swallow was the need for a "fall guy""
"The police have got to have somebody they can stick for those three murders ... The way to handle them is to toss them a victim, somebody they can hang the works on."
This is a central part of Spade's strategy. If he and Gutman can between them decide on a "victim", the police will accept it, and take the pressure off the others. It has to be part of the deal for handing over the falcon.
So, here we have Gutman - a notorious villain - and Spade - who is still himself a suspect in at least one of the murders, and we're asked to believe that the police (and subsequently a jury) will accept their word for who the real murderer is.
Doesn't sound very likely, does it?