A judge has dismissed the wrongful death lawsuit filed against Harvard University by the mother of a Cambridge man slain in a dormitory by nonstudents who used a Harvard student’s identification card to get into the building.

In a nine-page ruling, ­Middlesex Superior Court Judge Mitchell Kaplan sided with Harvard, which contended it was not liable for the actions of three New York men who lured Justin D. ­Cosby into Kirkland Hall and shot him to death in 2009 when their planned drug robbery turned violent.