A man accused of killing and dismembering a Perth drug dealer, whose head washed up on Rottnest Island, has told a court he acted in self-defence.

Aaron Carlino, 30, is on trial accused of murdering Stephen Cookson, 56, at an apartment in East Perth in December 2012 .

The Supreme Court has been told Carlino shot Cookson twice in the head as he slept, and then used an angle grinder and a knife to cut his body into six pieces.

The body parts were initially buried at a property at Golden Bay, south of Perth, before being dug up and dumped at sea.

Carlino pleaded guilty to unlawfully killing Cookson but denied the murder charge, with defence lawyer Laurie Levy telling the court he had "been subject to months of physical, psychological, emotional and financial abuse."

Prosecutor Carmel Barbagallo said Cookson was involved in both legal and non-legal activities, including drug dealing, and engaged Carlino to be his driver.

She said in an interview with police, Carlino said he moved into Cookson's apartment in East Perth after Cookson claimed he could help him get out of financial difficulty and "be a millionaire within 12 months".

"He agreed to work for Cookson and serve an apprenticeship to learn ways of making a lot of money very quickly and get out of debt," Ms Barbagallo said.

"He became Cookson's right-hand man, driver, and doing his shopping and laundry and assisting in his criminal activities."

However, Ms Barbagallo said Carlino told police he later realised money he had given to Cookson for investments was gambled away, and when he tried to leave Cookson's behaviour changed and he became more aggressive.

She said Carlino wanted out of the arrangement but "he believed there was no way out unless he killed him".

Killing 'cold-blooded, premeditated'

Ms Barbagallo said while Carlino claimed he was acting in "self-defence", that was rejected by the state, which maintained Cookson's death was "a cold-blooded, premeditated murder of a man as he slept".

In an opening address, Carlino's barrister Laurie Levy said at the time he shot Cookson, "Aaron Carlino had reached the point of no return ... he believed he had no other choice than to kill Stephen Cookson".

"By the time of Aaron Carlino pulling the trigger he had been subject to months of physical, psychological, emotional and financial abuse."

Mr Levy said Carlino was using a "not insignificant" amount of methylamphetamine at the time of Cookson's death.

He said Cookson was "charismatic" and had "beguiled Carlino with promises of making him wealthy".

But Cookson was also "a rather cunning, conniving person" who "defrauded people of their money".

"In effect he'd taken over Aaron Carlino's life," Mr Levy said.

"Every aspect of Aaron Carlino's life was controlled by Cookson, from his mobile phone, to whether he could have a girl stay over."

Carlino's cousin, Yovan Gonzalez, was also on trial accused of helping to dispose of Cookson's body parts at sea.

He pleaded not guilty.

His lawyer Paul Yovich told the court his client did not knowingly help Carlino dispose of the body.

The trial is set down for four weeks.