Dustin Gran started the sanity phase of his trial Wednesday. Last week, a jury found him "guilty" on multiple charges, including first degree murder.
Prosecutors say Gran killed Krista Pike, at the request of another teenage girl.
The trial was moved to Modesto because of the attention this case received in Madera.
The defense attorney told jurors a car accident forced Dustin Gran to have brain surgery that dramatically changed his personality and ultimately turned him into a killer.
A large scar remains on Dustin Gran's head from the surgery that's now a key piece of his sanity trial.
Defense Attorney John Garvin told jurors Gran had a small piece of his brain removed after a car accident in 2003 caused him to have seizures and strange daydreams.
Gran's mother testified the 11 hour operation changed him from a happy, lovable child to an angry, introverted teenager.
Garvin also said the surgery left him with no moral compass. In fact, he said Gran compared murdering Krista Pike to a ride at Disneyland because he had to wait a long time and the "fun" was over so fast. The prosecutor said there's no doubt Gran has mental issues, but that doesn't mean he's legally insane under California law.
ABC30 Legal Analyst Tony Capozzi said, "The standard for whether or not someone is insane is the old rule: 'Do you understand right from wrong?'"
A jury convicted Gran of first degree murder for attacking Pike with a dumbbell and then a knife. Investigators say he killed the 18 year old to please Brittany Navarra, 16, because she had romantic feelings for Pike's fiancé, Thomas Hollier.
If the same jurors decide Gran was sane at the time of the crime, he's facing life in prison. But if they feel he was insane, he'll be sent to a state mental hospital.
Several experts are scheduled to testify, but Capozzi says either way, he doesn't expect the 25 year old will ever be released.
"Especially with a violent crime like this, the public needs to be protected," explained Capozzi. "It's going to be difficult for this man to see the light of day again. I just don't see it."
The sanity phase of Gran's trial is expected to last about a week and a half.
Brittany Navarra is scheduled to start her trial in May, but it could be pushed back as late as August.