CHICAGO—Sammie Booker, sitting on the witness stand, revealed to jurors that he had robbed and extorted scores of drug dealers, killed three people, and aided at least two other murders as an underling of the Four Corner Hustlers gang on the West Side of Chicago in the past two decades. And more importantly, he said, he did so at the orders of his former boss Labar Spann, the lone defender in the trial. Booker’s testimony, which lasted three days this week, is a key move by the federal prosecutors to prove Spann’s alleged racketeering activities over two decades under the Racketeer Influenced and Corrupt Organizations Act (RICO). If found guilty by the jury, Spann faces up to life in prison. The attorney defending Spann, Steven Shobat, rose to shake Booker’s credibility by attacking his motive and character. Shobat reminded the jury that Booker could get drastically reduced sentences in exchange …