It came out in the trial.  Are you claiming the state fabricated the evidence?  They submitted the notebook and it was in his handwriting.

It was also clear that he expected the police to show up eventually after the shooting.  Hence, the armor.  I don't think he really expected to survive.  

Another thing he had in his notes was how far the theater was from the police station and his estimate of their time of arrival, as I recall.  As usual, they didn't go rushing in, but waited until the shooting ended.  Their job isn't to protect, but to apprehend and prosecute after the fact.  This has been upheld in courts numerous times.