Authorities say O'Donnell spent the hours before the December murder smoking crack with the mother of 14-year-old Ebony Dorsey.
The honor student was baby-sitting his 4-year-old child. Prosecutors say he then viewed pornography and sexually assaulted Dorsey.
Defense attorney Thomas Egan argued at the judge-only trial that O'Donnell was too intoxicated to form the intent required for first-degree murder. But, Judge William J. Furber sided with prosecutors after weighing five days of evidence and more than 100 exhibits.
The girl's mother, 34-year-old Danielle Cattie, pleaded guilty in May to drug and child endangerment charges. She is scheduled to be sentenced in October.
