A man fatally shot his former partner using a cartridge with her written name on it, a Perth court has been told.

Darren Kale Walters is on trial for the murder of 34-year-old Leah Anne Appleton, with a shotgun in West Perth, on August 3 last year.

He has pleaded guilty to manslaughter, but the State is continuing to try to prove the murder charge.

Prosecutor Nick Cogin said Walters killed Ms Appleton because he suspected she was having an affair.

Mr Cogin said Walters and Ms Appleton were both drug users, and Ms Appleton also sold drugs.

The pair had an "on-again, off-again" relationship and had two children.

At the time of her death, Ms Appleton had left Walters, who had assaulted her over suspicions she was having an affair with an acquaintance, Alex Thompson.

Walters an 'absolute aggressive jerk'

Mr Cogin said Walters assaulted Mr Thompson with a shotgun at a block of units on Mayfair Street, forcing him to phone Ms Appleton and ask her to return.

As Ms Appleton returned to the block in the early hours of August 3, Walters came down to the carpark with the firearm and "shot her at close range".

His intention to kill was "self-evident", Mr Cogin said, and the spent cartridge had Ms Appleton's name written on it.

But defence lawyer Linda Black said her client had "genuinely believed" there were no cartridges in the shotgun, which belonged to Jordan Dean, a resident of the block of units, and it was not easy to see if it was loaded.

Ms Black told the court that while Walters had taken drugs and was, behaving like "an absolute aggressive jerk", he "didn't write names on any cartridges".

The trial continues.