Richard Pratt QC, defending the teenager, also claimed that writings in the boy's diary - which, according to prosecutors, were indicative of a plan to murder students and teachers - represent the "wildest piece of fantasy".
The boy and a younger co-defendant, also 15, are on trial at Leeds Crown Court over accusations that they devised a plot to murder classmates and teachers at their school in Northallerton, North Yorkshire.
According to prosecutors, the pair, who were 14 at the time, had been inspired by Eric Harris and Dylan Klebold, the teenagers who killed 12 students and a teacher at Columbine High School, Colorado, 1999.
Jurors have previously heard how the older of the two boys conducted internet searches on deadly weapons, with prosecutors claiming he "revelled" in the Columbine massacre.
Mr Pratt said: "Researching the Columbine murderers does not make you a killer. Buying guns and explosives - that's what makes you a killer.
"Of course, they did not kill, as Eric Harris and Dylan Klebold did, but neither did either of them make a real plan to murder."
Summing up the case for the boy's defence on Monday, he claimed there was no way he could purchase the "articles of destruction" necessary for the supposed attack, as he had, at most, £89 in his bank account.
"(The boy's) activities on the internet have led the prosecution to claim that this 14-year-old was acting as a terrorist," Mr Pratt told jurors.
"But there's some difference between (the boy) and a terrorist, isn't there?
"They require the means by which to carry out their deadly deeds. They do not pause to window shop, they go in and they buy."
Jurors had previously heard extracts of the diary which allegedly revealed a plan to murder his former girlfriend's parents and become a "natural born killer".
Mr Pratt said: "Is that real? Is that the exposition of a suicidal, homicidal person? Or is it simply the wildest piece of fantasy?"
Earlier in the day, prosecutor Paul Greaney QC had claimed the defendant's refusal to take to the witness box showed the boy and his co-accused could not "answer the prosecution's case".
However, Mr Pratt reminded jurors the older defendant had been interviewed by counter-terrorism officers, which was written up into an 145-page script.
He added: "I do not suppose anyone, be they young or old, saint or sinner, would wish to be cross-examined by Mr Greaney."
The defendants deny a joint charge of conspiracy to murder, as well as alternative charges of intentionally encouraging or assisting an offence and encouraging or assisting an offence believing it would be committed.
The older boy additionally denies unlawful wounding over claims he carved his name into his former girlfriend's lower back, and aggravated burglary after supposedly breaking into her parents' home with a knife.
The trial continues on Tuesday when the defence case for the younger boy is expected to be outlined.