(CNN) -- James Holmes for years thought of how to kill a large number of people, according to a notebook he kept before a 2012 mass killing in a Colorado movie theater.
"The obsession to kill since I was a kid, with age became more and more realistic," he wrote in the notebook, which he sent to a psychiatrist before the assault and which became part of the evidence in Holmes' mass murder trial Tuesday.
The book is seen as a key piece of evidence. Prosecutors are trying to prove Holmes meticulously planned his crime, while the defense has said he was having a psychotic episode when he opened fire at the premiere of the Batman movie "The Dark Knight Rises."
Aurora Police Sgt. Matthew Fyles read excerpts from the notebook on Tuesday, some of the words describing how Holmes listed pros and cons of several different types of attacks and venues.
He ruled out striking an airport because he didn't want people to think he was a terrorist.
"Terrorism isn't the message," he wrote. "The message is, there is no message."
Holmes wrote that people would erroneously believe the failures with women or with jobs led to the July 20, 2012, rampage that took the lives of a dozen people and wounded scores more.
"Both (failures) were expediting catalysts, not the reason. The causation being my state of mind for the past 15 years," he wrote.
Holmes, 27, a former doctoral student in neuroscience, is standing trial on charges of capital murder and other offenses. He is accused of killing 12 people and injuring 70 more. He faces 165 counts. Holmes has pleaded not guilty by reason of insanity. Prosecutors have said they will seek the death penalty.
While the prosecution had Fyles talk about the plans for the attack in the notebook, the defense focused Tuesday on Holmes' existential thoughts and questions. He asked about the meaning of life and the meaning of death, and at other times he wrote the question "Why?" over and over.
In one part of the notes, Holmes talks about his "divided" self.
"The real me is fighting the biological me," he wrote.
He said the first obsession with killing came more than 10 years before, then he discussed how he fought his thoughts with no success.
"So anyways, that's my mind. It is broken. I tied to fix it. I made it my sole conviction but using something that's broken to fix itself proved insurmountable," he wrote.
Holmes used the notebook to discuss the different ways to kill.
He first wrote of fantasizing about using nuclear bombs or a biological agent.
"Most recently (I considered) serial murder via a cellphone, stun gun and folding knife at national forests," he wrote.
Holmes decided instead on an attack at a mass gathering despite knowing there was a "99 percent" chance he would be caught.
In the notebook, Holmes diagrammed the individual theaters and even noted that a police station was just three minutes away. Holmes apparently visited the theater two weeks before the night of the shootings, a detective testified in 2013.
Holmes listed three options on how to conduct the shooting, when to begin the attack and weighed the possibilities. Holmes considered theaters 12, 10 and the theater he ended up in, 9.
Fyles said the notebook began on Page 27 and is 29 pages long.
The notebook was initially addressed to his mom, dad and sister, but he eventually sent it to his University of Colorado psychiatrist, Dr. Lynne Fenton.