Story highlights Notebook is key piece of evidence in which James Holmes discusses motive, possible methods and wonders about life's meaning

Holmes said he first considered killing people more than 10 years before July 2012 attack

His lawyer focuses on passages in which he talked existentially and how he fought his "divided" self

(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.

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He ruled out striking an airport because he didn't want people to think he was a terrorist.

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