A Roxbury man pleaded not guilty to illegal gun possession charges Thursday stemming from his arrest by Boston police following a botched armed home invasion in Dorchester that left one of the intruders dead. Cedrick Slayden, 18, was ordered held on $50,000 cash bail and had his bail revoked an earlier larceny charge. Police and Suffolk District Attorney Daniel F. Conley’s office continue to investigate an Esmond Street armed home invasion that ended when the resident shot and killed one of the masked intruders. Police say that when Slayden was arrested after the incident he was armed with a pistol. He is due back in court Feb. 3.