A murder trial witness whose evidence helped jail three killers has been exposed as a violent rapist.

Shaun Sinclair has been convicted of raping two women, attacking a third and having sex with another 15-year-old girl.

He raped one woman after lining up knives on a table and asking her what one she "wanted to die with".

The 23-year-old was found guilty following a trial at the High Court in Glasgow.

He returned to the court two years after his testimony led to three people being jailed for the torture and killing of a vulnerable man in North Ayrshire.

Sinclair was a vital prosecution witness in describing how Scott Blackwood, 30, was brutally attacked before attempts were made to set him on fire.

Allan Nicol, 36, and Thomas Moore, 26, were locked up for murder while Georgina Smith, 22, was jailed for culpable homicide.

Sinclair now faces his own lengthy spell in prison when he is sentenced later in August.

The jury heard how he raped a woman in April 2014. He had earlier had sex with her when she was only 15.

He carried out the sex attack after initially ordering her out a flat in the south side of Glasgow before demanding she return.

When he met her, Sinclair chillingly told her: "You're going to get a shock when you go in."

The victim said she knew "something bad was going to happen".

As she entered, all the kitchen knives were spread on the table and Sinclair told her to pick the one she wanted to die with.

In his speech to the jury, prosecutor Bruce Erroch said: "What calculating behaviour ... I say it borders on sadism."

The woman recalled Sinclair punching and trying to choke her. Struggling for breath, Sinclair went on to rape her.

The court heard Sinclair had "battered her black and blue to overcome her will".

Sinclair then carried out a separate sex attack that day leaving her in tears.

The woman did go to the police but Sinclair made her withdraw the allegation.

It was put by Sinclair's lawyer the sexual violence did not happen but the distressed victim replied: "So, I gave myself two black eyes?"

He went on to rape her again on Christmas Eve 2014 after leaving her to go and have sex with a neighbour.

She recalled him coming into their Ayrshire flat looking "dead vicious ... making gestures as though he was going to kill me".

The jury heard he dragged her by the hair yelling: "You are making me do this" before holding a knife to her throat then raping her.

The court heard she was left with "a catalogue of injuries".

Sinclair also raped another woman at a house in Girvan, South Ayrshire, in January 2013.

He attacked her after smashing up the property. The woman had tried to calm him down, but, instead, Sinclair grabbed her wrists and forced himself on her.

Sinclair was convicted of having unlawful sex, assault and rape in relation to the first woman.

He was found guilty of raping the second woman as well as physically assaulting a third female at a flat in Largs, North Ayrshire, in April 2012.

Sinclair was further found guilty of having sex with another 15 year-old girl in 2013.

Judge Norman Ritchie QC deferred for reports and put Sinclair on the sex offenders register.