Experts are questioning whether courts worldwide are giving consistent sentences to hackers following news that a Japanese man has escaped jail, despite admitting writing a virus that wiped music and movie files on innocent users' computers.

Masato Nakatsuji, who was revealed to be the first ever virus writer to be arrested in Japan when he was apprehended in January, admitted writing the malware which displayed images of popular TV anime characters while destroying data on third party computers. The malicious code was spread via the controversial Winny file-sharing system in Japan last year.

Today, Nakatsuji, a graduate student at Osaka Electro-Communication University, was found guilty in Kyoto District Court and sentenced to two years in jail. However, as the sentence is suspended for three years he will not have to serve any time in prison.

"Masato Nakatsuji has been found guilty of copyright infringement rather than for the damage his movie and music-munching malware caused," said Graham Cluley, senior technology consultant for Sophos. "One has to wonder whether if he had been apprehended in another country then he would have been charged with a more conventional cybercrime and might have got a more serious sentence."

Nakatsuji has claimed that he wrote his malware to try and punish people who downloaded copyrighted material from peer-to-peer file-sharing networks.

"If movies and animated films are illegally downloaded, TV networks will stop showing these programs in the future," Nakatsuji said during the trial, trying to explain his behavior. "My hobby is to watch recorded TV programs, so I was trying to stop that."

"There are enough cybercriminals out there causing harm and stealing money and identities with malicious code -- the last thing we need are vigilantes entering the mix, writing malware to try and put right what they believe to be wrong," says Cluley. "If someone sees the law being broken on the net -- go to the authorities. Don't write malware. This man is frankly lucky to have got away without a more serious punishment."

Coincidentally, Isamu Kaneko, the author of the Winny file-sharing program, was fined by a Japanese court in December 2006 for assisting in copyright violation. The rights and wrongs of the case have been widely debated on the Internet.