Stewart was giving evidence at Belfast Crown Court against the fourteen men he and his brother David implicated in a catalogue of UVF terrorist crime.
He agreed with a second defence lawyer, Barry Macdonald QC, that he was "a right nasty piece of work", but denied being "corrupt to the core."
He claimed that the only thing that "sets me free is the truth."
The 37-year-old rejected earlier defence suggestions by QC Frank O' Donoghue, representing former UVF leader and police agent Mark Haddock, that he was a liar motivated by opportunistic self-preservation and greed.
Stewart, who has already confessed to UVF membership and playing a role in the loyalist killing of Tommy English in 2000, has turned Queen's evidence along with his brother against his alleged terrorist associates.
Mr Macdonald, for the first of Haddock's co-accused, and allegedly his second-in command, 39-year-old David 'Reggie' Miller, began his cross-examination by asking him how his client could be in a flat plotting a murder, while police had him six miles away.
Later, Stewart admitted at the ages of 20, 21 and 24 that he had had sex with under-aged girls.
When asked by Mr Macdonald if he had "preyed on vulnerable people", robbing milkmen, and fast-food delivery men, Stewart readily agreed.
He claimed: "Yeah, I think we were all had a go" as he "wasn't doing these things on my own."
Stewart reluctantly agreed he had a bad character, but he rejected out of hand the suggestion that he was "corrupt to the core."
"I am obviously not a nice person," said Stewart, before agreeing that he was "a rotten egg at times" and ultimately, "a right nasty piece of work."
Mr Macdonald also put it to him that the only reason he had given himself up to the authorities was because he was "on the run from the UVF and saw a way out.
"You wanted a deal with police, and wanted to trade the information you had in exchange for protection," he said.
Stewart denied this, but said that logically he would "obviously need protection."
He also denied making a deal with the authorities.
Stewart, who is due to be cross-examined again on Friday, stated: "To be honest with you my whole life has stunk ... I wish I had done something else .... but I am trying to make it right now".
He claimed that the turning point in his life came as he held a taxi driver hostage and hearing the siren of an ambulance rush by on the night UDA rival Tommy English was shot dead in October 2000.
His life, added Stewart, "changed from that moment on ...I agree I was a nasty piece of work... but that was a defining moment."
It was defence QC Frank O'Donoghue who initially questioned Stewart about his motivation.
Mr O'Donoghue told Stewart that he wanted to suggest to him that he is "a terrorist or may well have been a committed terrorist and you may well have knowledge, because of your own role in these events.
He then told Stewart that his assertions about the movements of Haddock's BMW car in relation to English's murder were "entirely inconsistent" with evidence disclosed to the defence by the prosecution.
Earlier, trial judge Mr Justice Gillen threatened to clear the Belfast Crown Court after there was applause and cheering from the dock and public gallery.
This came when Stewart admitted he'd made a mistake in his evidence.
Mr Justice Gillen warned that if there was even one more outburst, he would "without hesitation" order the court to be cleared.
Stewart's brother David has yet to be cross-examined.
Nine of the men in the dock, including Haddock, deny the murder of Tommy English and numerous other terrorist related charges.
He agreed with a second defence lawyer, Barry Macdonald QC, that he was "a right nasty piece of work", but denied being "corrupt to the core."
He claimed that the only thing that "sets me free is the truth."
The 37-year-old rejected earlier defence suggestions by QC Frank O' Donoghue, representing former UVF leader and police agent Mark Haddock, that he was a liar motivated by opportunistic self-preservation and greed.
Stewart, who has already confessed to UVF membership and playing a role in the loyalist killing of Tommy English in 2000, has turned Queen's evidence along with his brother against his alleged terrorist associates.
Mr Macdonald, for the first of Haddock's co-accused, and allegedly his second-in command, 39-year-old David 'Reggie' Miller, began his cross-examination by asking him how his client could be in a flat plotting a murder, while police had him six miles away.
Later, Stewart admitted at the ages of 20, 21 and 24 that he had had sex with under-aged girls.
When asked by Mr Macdonald if he had "preyed on vulnerable people", robbing milkmen, and fast-food delivery men, Stewart readily agreed.
He claimed: "Yeah, I think we were all had a go" as he "wasn't doing these things on my own."
Stewart reluctantly agreed he had a bad character, but he rejected out of hand the suggestion that he was "corrupt to the core."
"I am obviously not a nice person," said Stewart, before agreeing that he was "a rotten egg at times" and ultimately, "a right nasty piece of work."
Questioned about his motivation for going to the police, Stewart said when he did so he had been a "terrorist for 14 years... but there was no shining light that came down."There's no house in Malibu, if that's what you're asking... I haven't gained anything.Robert Stewart
Mr Macdonald also put it to him that the only reason he had given himself up to the authorities was because he was "on the run from the UVF and saw a way out.
"You wanted a deal with police, and wanted to trade the information you had in exchange for protection," he said.
Stewart denied this, but said that logically he would "obviously need protection."
He also denied making a deal with the authorities.
Stewart, who is due to be cross-examined again on Friday, stated: "To be honest with you my whole life has stunk ... I wish I had done something else .... but I am trying to make it right now".
He claimed that the turning point in his life came as he held a taxi driver hostage and hearing the siren of an ambulance rush by on the night UDA rival Tommy English was shot dead in October 2000.
His life, added Stewart, "changed from that moment on ...I agree I was a nasty piece of work... but that was a defining moment."
It was defence QC Frank O'Donoghue who initially questioned Stewart about his motivation.
Mr O'Donoghue told Stewart that he wanted to suggest to him that he is "a terrorist or may well have been a committed terrorist and you may well have knowledge, because of your own role in these events.
He then told Stewart that his assertions about the movements of Haddock's BMW car in relation to English's murder were "entirely inconsistent" with evidence disclosed to the defence by the prosecution.
Earlier, trial judge Mr Justice Gillen threatened to clear the Belfast Crown Court after there was applause and cheering from the dock and public gallery.
This came when Stewart admitted he'd made a mistake in his evidence.
Mr Justice Gillen warned that if there was even one more outburst, he would "without hesitation" order the court to be cleared.
Stewart's brother David has yet to be cross-examined.
Nine of the men in the dock, including Haddock, deny the murder of Tommy English and numerous other terrorist related charges.
