A prison officer from Corby contacted the press about James Bulger killer Jon Venables because of the "complete injustice" of his special treatment behind bars, a court heard.
Scott Chapman is on trial at the Old Bailey accused of accepting £40,000 in exchange for tips about Venables while he was in jail for child porn offences in 2010.
He was allegedly the source of a string of stories in The Sun, News Of The World, Daily Mirror, Sunday Mirror, People, Daily Star and Daily Star Sunday.
Giving evidence in his defence, Chapman broke down in tears as he told jurors why he first contacted The Sun.
He felt strongly about improvements at the prison where he worked because of his guilt over the suicide of an inmate on his watch shortly after he had arrived there in 2003, the court heard.
All inmates were equal and entered the system in a "safe, secure and relaxed environment" – even serial killers, famous footballers and terror suspects.
But when Venables arrived without warning in March 2010, the child killer was treated differently and that had a "huge impact".
Chapman said: "I phoned the newspaper because I thought everything that was happening was wrong.
"It was impacting on the good work we had been doing at the prison. It was impacting on the smooth operation of where I was working.
"My motivation was to try to highlight what was happening, to stop it happening."
Asked by his lawyer Paul Mendelle QC whether money entered into the conversation, he said: "Yes, (The Sun journalist) raised the topic of money and payment and things like that.
"The gist of what he was saying was every story that was printed in the paper had to have a source and somebody had to get paid."
He went on to say the reporter told him it was not illegal, but just "the way papers run".
Asked if he had rung up wanting to be paid, the father-of-one said: "Payment never entered my mind. I just rang up with a complete injustice at the time. I just think it's wrong."
He told the jury he thought it might lead to disciplinary proceedings if he got caught leaking information to The Sun, but another more senior reporter who became his main contact, told him: "While you are an active source you are a protected source."
Chapman said he started phoning other newspapers after The Sun journalist began "pestering" him on the phone, making him feel "trapped".
He said: "I remember him saying the only way a source becomes no good to The Sun is if the information gets duplicated in other papers, because that's what they are fighting for. I thought I would test the theory."
After a while money also became a motivation, he said: "The overall total sounds staggering. I'm human. It shocked me. Money they were chucking at me – £2,500 for a five-minute conversation."
Chapman was dismissed from his job on "medical grounds" in April 2011, a few months after his personal phone was seized at the prison and the information on it removed.
He went on talking to newspapers but only passed on recycled details on Venables that had already been printed, he said.
Chapman, 42, and his ex-partner Lynn Gaffney, 40, of Corby, Northamptonshire, deny misconduct in a public office.
Sunday Star reporter Tom Savage, 37, of south London, and a News Of The World journalist, who cannot be named for legal reasons, deny conspiring to commit misconduct in a public office.