The Feb. 12, 2008, killing took place in Oxnard, but the case is being tried in the Chatsworth section of Los Angeles.
In her opening statement, Ventura County Deputy District Attorney Maeve Fox said McInerney, now 17, and other boys bullied the 15-year-old King. But shortly before his death, a more confident King had begun wearing high heels, makeup and earrings to school.
"Larry King for the first time in his life wasn't taking it anymore," Fox said.
The day before King was shot, "Brandon said, 'I am going to shoot him.' And this is what a student will testify to," Fox said.
Prosecutors have added a hate-crime count, alleging McInerney had neo-Nazi sympathies.
Defense lawyer Scott Wippert said King harassed McInerney, his effeminate behavior encouraged by school administrators.
Wippert said McInerney was not a white supremacist but had such materials because he was working on a paper on Hitler.
A voluntary manslaughter conviction would make McInerney eligible for release before age 40.
Notable deaths of 2014 [PHOTOS]
2 killed when a tour bus crashes in Delaware