Owen Thor Walker, 18, also known online as 'Akill', pleaded guilty to several charges of interfering with computer systems but the case was discharged so that he will not have a criminal record.
The court said that it had taken the unusual step so that Walker could work with local police in fighting online crime.
Police have said that, while Walker did not take money directly from people's bank accounts, he was paid US$30,000 by persons unknown to write the malware.
The court fined him just US$11,000 and seized the computers he had bought with his fee.
Both the prosecution and the defence teams argued that he should be allowed to walk free and work with the police in the future. Justice Judith Potter said that Walker was a young man with a potentially outstanding future in law enforcement.
Walker was caught as part of a crackdown by the FBI on botnets, the networks of infected computers that can be used to harvest banking data or in spamming or denial of service attacks.
Eight other people have been charged as part of the investigation.
The news of Walker's freedom will anger some in the computer security industry, who see the decision as unethical and likely to encourage others to follow in his footsteps.
Antivirus firms, in particular, never hire virus writers. "It is not something any reputable firm does," said Graham Cluley, senior technology consultant at Sophos.
"The industry is very clear that it does not hire criminals. In the case of antivirus they probably do not even have the skills we need."
Details of the group Walker had been working with have not been released since more arrests are expected shortly.
Kiwi hacker walks free from court
By Iain Thomson on Jul 17, 2008 7:52AM