The Supreme Court was told Anthony Prestidge had just arrived in Perth for a holiday and found out Andy Ball was beating his sister.
He bludgeoned Mr Ball, 24, to death and then fled to Bali.
Prestidge remained at large until he was found living in Thailand where he came to the attention of Interpol after he was arrested for minor immigration breaches.
He was then extradited to Perth.
Prosecutors told the court Mr Ball had said to friends he had been receiving threats and that if anything happened to him, it was his brother-in-law who was responsible.
Prestidge has always denied any wrongdoing claiming he had acted in self defence because Mr Ball had come at him with a knife.
Justice Jenkins said the offending was mitigated by his emotional stress but aggravated by the fact he fled the country.
The sentence will be backdated from May 2011 when he was extradited.