Classes were back in session Thursday at UCLA as the university mourned a professor who was remembered as brilliant and kind, one day after he was shot dead by what authorities said was a disgruntled student. Colleagues and friends of William S. Klug described the mechanical and aerospace engineering professor to reporters as a “supportive guy.” He was one of the two men found dead in an engineering building office Wednesday morning. Investigators determined Klug was the “murder” half of a murder-suicide likely perpetrated by the student, according to multiple sources. Klug was a married father of two young children and described as a rare gentle soul in the competitive world of academic research. “I am absolutely devastated,” said one colleague. “You cannot ask for a nicer, gentler, sweeter, and more supportive guy than William Klug.” Officials have not yet identified the shooter.