"You killed First in order to get at Joe Slovo," George Bizos SC, who is appearing for the Slovo family, told Williamson.
Williamson denied that he had been frustrated at the failure of attempts to kill the former SA Communitst Party leader, but later he conceded that he had been exasperated by Slovo's ability to elude the security force strikes against him.
The former apartheid spy is applying for amnesty for his part in arranging the letter bombs that killed First in Mozambique in 1982 and Jeanette Schoon and her six year-old daughter Katryn in Angola in 1984.
Williamson said Slovo was one of the former government's most sought after enemies, but admitted: "I had a sneaking admiration for the grey fox."
He said Slovo had been extremely vigilant and constantly moved around Maputo to avoid detection by security force raids on his home.
"I had the greatest respect for how he evaded all efforts to kill him," Williamson said.
The spy admitted to arranging to have explosives inserted into a letter intercepted en route between Lesotho and Mozambique, but claimed he was not certain to whom it was addressed.
He told the TRC's amnesty committee he recalled his superior, the late Brigadier Piet Goosen saying the letter was for Slovo, but said this could have meant either Ruth First or Joe Slovo.
Williamson told the committee he had no doubt that the "upper echelons" of the security forces knew of his actions and supported them. He said he was convinced that the whole counter revolutionary strategy employed at the time was orchestrated by the State Security Council.
He also spoke of the fear and anxiety that existed amongst security policemen when it appeared that the African National Congress would come to power.
"If anyone thinks it easy to see the enemy become the government and things you have been shedding blood for decades to prevent, then they have no understanding of human behaviour. It is a difficult and painful situation," he said.
He said many of the security officers had doubts about the future and were afraid of what might happen.
However, in spite of this many security policemen remained and now serve the present South African government.
"I did not. I never asked for a big job, although I believe I have something to contribute..." he said.
Asked by Bizos about the bomb attack which killed Jeanette and Katryn Schoon, he said he believed she and her husband, ANC activist Marius Schoon, had been teaching English to Cuban soldiers in Angola.
He claimed that this could have been useful to the Cubans in their manning of air defence installations in the Lubango area of Angloa, where the Schoons lived.
Asked by Bizos where he obtained this information, he replied that it had been passed on to him by SA Air Force intelligence.
He explained that although he had been friends with the Schoons while at the University of the Witwatersrand in the early 1970s, he still viewed them as the enemy because of their involvement with the ANC.
However, he said he did not know that the Schoon's children were with them in Angoloa at the time.
"For me it made absolutely no difference if Joe Slovo or Ruth First or Marius or Jeanette Schoon were killed by the bombs, but I never deliberately targeted childen," Williamson said.
Marius Schoon and his son Fritz, who was two at the time and survived the blast, have been attending the amnesty hearing in Pretoria. So have Slovo's children Gillian, Shawn and Robyn.
During the hearing, transcripts of conversations Williamson had with Gillian Slovo in 1995 after he admitted involvement in the murder of her mother were handed to the committee.
In one paragraph Williamson told her that her parents had been targeted because of a perception that existed amongst security forces that the ANC and SACP could not function properly "without white brains".
"It was a basic fundamental belief that... if it weren't for the `blanke kommuniste' (white communists) then we wouldn't actually have this problem we've got," Williamson said.
The hearing continues on Friday.