One of two defendants in a Petaluma murder case was sentenced to 15 years to life in prison this morning in Sonoma County Superior Court.Keith David Kellum, 24, of Santa Rosa, pleaded guilty to second-degree murder in March in connection with the February shooting death of 20-year-old Uriel Arango Carreno in Carreno’s Petaluma home. The term was part of a plea agreement. Kellum’s co-defendant, Bradley Blackwell, 19, of Rohnert Park, was convicted of first-degree murder and of attempted robbery and burglary. A pre-sentence diagnostic exam report on Blackwell is due in August. Kellum said he and Blackwell went to Carreno’s residence, a detached, converted garage at 1429 Joan Drive, on Feb. 7 to “rip off” a drug dealer. “I kicked the door in and Mr. Blackwell shot a man,” Kellum told Judge Rene Chouteau when he pleaded guilty to the murder charge.Defense attorney Martin Woods said Kellum has more than 700 days’ credit for time served.