The book, which was seized during a police search at the teenager's home in October 2017, also describes the human race as a "vile species which needs to die out".
-> Also in court: Ring and bring crack cocaine and heroin dealers jailedOne passage reads: "Everyone is filthy and deserve to be shot, including me. I'll play the role of god and decide who a let live and die (sic)."
"The Human condition is a curse and burden," it adds.
The boy, who is co-accused with a friend of plotting to kill innocent people at their school in Northallerton, North Yorkshire, said he owned and solely wrote the book during a police interview which was conducted on the same day it was found.
On Friday, jurors at Leeds Crown Court heard extracts from the interview, in which the boy claimed the diary "was not intended for anyone else to see".
"You have to understand that it was all hypothetical, just thoughts and feelings," he said.
He admitted he had written about how he would "obliterate" his school, and had assembled a list of "stuff we need", including napalm, firearms and pipe bombs.
-> Police still at scene of potential shooting in Leeds as investigations continue into Burmantofts Street incidentHowever, he claimed he wrote such things to get them out of his head, and said parts of the diary reflected his "fascination" with the "morbid psychology" of prolific killers.
He told a police officer: "It was an admiration of these killer type people - you know, when people die and you find their diaries.
"I was not going to do any of it, it was all an admiration type of thing."
Discussing the detailed nature of some of the diary entries, he said: "It was just getting stuff out of my head, and they are basically a sort of therapy."
Jurors were also read a passage where he claimed to "love and hate" the ideology of Adolf Hitler and supported the "weaning out of the weak".
However, the boy said he wrote certain bits to "annoy people" and fit in with the supposed "dark, morbid humour" of his classmates.
During his own police interview, the younger defendant, who was also aged 14 at the time, said he believed his co-defendant was "joking" when he discussed the alleged plan, and felt "scared and isolated" as it started to get more serious.
"(The older defendant) ripped out the map page of his planner and said we should put bombs in these rooms and blow up the school," the boy said.
"He said I would just stand at the gates and mow people down. I was scared at this point, because I could not go through with anything like this."
The boys jointly deny conspiracy to murder, encouraging or assisting an offence, and assisting or encouraging an offence believing it would be committed.
The older teenager has also pleaded not guilty to unlawful wounding and aggravated burglary.