Williamson is applying for amnesty for the letter bomb murders of Ruth First in Mozambique in 1982 and of Jeannette Schoon and her six-year-old daughter Katryn in Angola in 1984.
First was the wife of former SA Communist Party leader Joe Slovo and the Schoons were the wife and child of African National Congress member Marius Schoon.
Williamson admitted on Monday that in both attacks he had arranged to have the explosives placed in letters intercepted by the security police and addressed to the victims.
He told the TRC's amnesty committee in Pretoria that he had often told staff who worked under him that the best way of dealing with enemy targets was to "dehumanise" them.
"I don't believe that people could have done what we did if they saw the enemy as invidual human beings," he said.
Williamson said he had experienced great difficulty in coming to terms with the deaths of Jeannette and Katryn Schoon because he had known the family well while with them at university, where he worked under cover as a police spy.
Showing faint signs of emotion, he said hearing of the death of the child was like "being hit in the face with a bucket of cold water" because he did not know the Schoon children were with their parents in Angola.
Williamson justified the attack on the Schoons on the grounds that they were an important link in the ANC network in Angola.
Earlier he described how the bomb that killed First was prepared. He said 100g of explosives was inserted into a letter intercepted between Lesotho and Mozambique.
Williamson said the letter bore the name Slovo but he could not remember if it was addressed to Joe Slovo or Ruth Slovo or to both at the Eduardo Mondlane University in Maputo.
He said the letter was sent to him through security police internal mail with an instruction that he should discuss the contents with his superior officer, Brigadier Piet Goosen.
He was then asked if he could arrange to fit an explosive device into the letter without materially changing its shape and weight.
Williamson said he asked explosives expert W/O Gerrie Raven to insert the bomb into the envelope, which bore the logo of an international organisation, possibly the United Nations.
He said Raven later returned with the envelope and said the bomb had been successfully fitted.
"I then asked him if he meant this letter could blow up the office and kill us both. When he replied that it could, this made me feel uncomfortable and I told him to get rid of it because I didn't want it there," he said.
Williamson said he later heard that Ruth First had been killed in an explosion at the university in Maputo.
He said the death of First was later noted at a security police meeting without referring to police involvement.
"Brigadier Goosen looked directly at me and nodded slightly," Williamson said, adding that he assumed Goosen was acknowledging that it had been the result of the letter bomb.
He said he was again asked by Goosen to have a bomb made to be sent to Schoon in Lubango in Angola. The same procedure was followed but the bomb only detonated about six months after it was sent by the security police.
During his testimony Williamson read at length from documents describing South Africa's struggle with the ANC and its strategies in countering the revolutionary onslaught.
He said he was never in any doubt that he was acting in terms of a counter revolutionary strategy when he sent the letter bombs.
Williamson's advocate Allan Levine then showed the first of a series of video recordings from television programmes depicting scenes from the years of conflict in South Africa. The video showed footage from the SABC programme Network in 1986, soon after a series of cross-border raids against ANC bases in neighbouring countries.
The hearing continues.