A career criminal has admitted to shooting dead self-described vampire gigolo Shane Chartres-Abbott in Melbourne in 2003 as a favour to an acquaintance.

The man is the crown's key witness and cannot be identified because of a number of suppression orders from Victoria's Supreme Court.

Mr Chartres-Abbott, a 28-year-old male prostitute who claimed he was a vampire, died after being shot outside his home in Reservoir on June 4, 2003.

Warren Shea, Mark Perry and Evangelos Goussis are the three men on trial for his murder.

Via videolink, the unnamed witness told the jury he was responsible for the death of Mr Chartres-Abbott.

In testimony on Wednesday, the court heard that just a few weeks before the killing, Shea told the unnamed key witness that Mr Chartres-Abbott had raped and maimed Perry's ex-girlfriend.

"I was told that he [Chartres-Abbott] made a shocking mess of her, bit hunks out of her,'" the witness told the court.

"I was told quite a fair bit about that, and [Chartres-Abbott] raped her and left her for dead."

The witness also told the court he was asked by Shea if he knew anyone who "would be interested in... harming Chartres-Abbott".

"I definitely said I'd take care of it personally," the witness added.

The witness testified that he accepted no payment for the killing, calling it a "favour for a favour".

Initially he had the wrong address for Mr Chartres-Abbott, but was eventually given the correct address by a policeman he knew. He wrote the details down on a TAB ticket.

The witness gave evidence that the third defendant, Goussis, drove him to Mr Chartres-Abbott's house on the day of the killing.

The prosecution alleged the three defendants engaged in joint criminal enterprise because they all intended that the murder be committed.

The three defendants have all pleaded not guilty.

The key witness will continue his evidence on Thursday.