The teen, described as a "geeky, uncoordinated, awkward 16-year-old," allegedly brought a carving knife to school, picked a victim at random in a boys' bathroom, then stabbed him eight times.
The teen's defense attorney told jurors that Odgren was mentally ill when he killed 15-year-old James Alenson in Jan. 2007.
Attorney Jonathan Shapiro said Odgren was struggling with a form of autism, attention deficit hyperactivity disorder, depression, anxiety and possibly bipolar disorder when he attacked the freshman, whom he had never met.
The different portrayals of Odgren, now 19, came during opening statements in Odgren's first-degree murder trial in a Woburn, Mass,. courtroom, about 15 miles north of Boston.
Assistant District Attorney Daniel Bennett said Odgren downloaded and watched a violent video the morning of the killing, took a carving knife to school, and waited in a school bathroom. When no one came in, he went to a second bathroom, where he and Alenson walked in at the same time.
Another student who was in a bathroom stall heard someone yell, 'What are you doing? You're hurting me. Stop!"' Bennett said.
Alenson then stumbled out of the bathroom and collapsed on the floor.
Bennett argued that when teachers and administrators gathered around Alenson to try to help him, Odgren said, "I just exploded."
Bennett said a student heard Odgren say, "I didn't mean to kill him. It was a mistake."
Bennett described Odgren's behavior when he was questioned by police, saying that when a state trooper told him Alenson had died, he started to cry, but within a minute asked the trooper, "What do you get for manslaughter - 10 to 15 (years)?"
The defense portrayed Odgren as a troubled teenager who had suffered from mental illness and developmental disabilities since he was a young child. Shapiro said that Odgren does not deny that he killed Alenson, but that no one may ever know why he did it.
"Why did a geeky, uncoordinated, awkward 16-year-old who had never been in any trouble with the law suddenly and without provocation ferociously stab to death a 15-year-old classmate who he did not even know?" Shapiro asked.
Bipolar disorder runs in Odgren's family, Shapiro said. His mother, Dorothy, has the disorder, and four of her relatives committed suicide, Shapiro said.
Odgren became obsessed with Stephen King novels, violence, crime and forensics. He also developed an "irrational fear that something was going to happen to him," Shapiro said.
He started bringing weapons to school, once a knife and another time a toy gun, Shapiro said. By January 2009, Odgren was "consumed by his delusions," Shapiro said.
He said that after Odgren stabbed Alenson, a student who had been in the bathroom heard Odgren blurt out, "Oh, my God. What have I done? What have I done?"