Incorrect. The *OWNER* of that slave was that *LIBERAL* representative of trouble making Massachusetts. (Still making trouble today.)
Even Wikipedia tells a teeny bit of the truth on this matter.
"In 1850, Irene Emerson remarried and moved to Springfield, Massachusetts. Her new husband, Calvin C. Chaffee, was an abolitionist. He was elected to the U.S. Congress in 1854 and fiercely attacked by pro-slavery newspapers for his apparent hypocrisy in owning slaves. "
Chaffee could have freed him at any time, but chose not to do so because he expected the case to better serve as a propaganda tool, or a judicial activism tool.
By that point, the case had been going through the courts for some years, financed by the children of one of Scott’s earlier owners who had turned against slavery. They lived in Missouri. So no, the case wasn’t hatched in Massachusetts, and Chaffee wasn’t any kind of mastermind.
Chaffee (or his wife) could have stopped the case by freeing Dred Scott, but arguably they would have faced more flak for making the case moot than they would have for owning a slave. After the decision, they did turn over the Scotts to the Blows who had financed the case and Henry Taylor Blow freed them.