The former Sinn Fein president made the comments while appearing as a witness at a trial of the facts at Belfast Crown Court.
The trial examined the evidence against veteran republican Ivor Bell, who was accused of two charges of soliciting the murder of Jean McConville.
The mother of ten was abducted, murdered and secretly buried by the IRA in 1972.
The evidence against Mr Bell was based on an interview that interviewee Z gave to the Boston College oral history of Northern Ireland’s troubled past, called the Belfast Project.
However Mr Justice O’Hara ruled the tapes were unreliable and could not be used as evidence against him.
On Thursday the judge directed the jury to find Mr Bell not guilty.
Claims by Z, about whom the judge said there was overwhelming evidence to prove was Mr Bell, included that Mr Adams was the officer commanding (OC) of the IRA in Belfast in December 1972.
Mr Adams has consistently denied he was ever a member of the IRA.
While appearing at the trial, Mr Adams described the IRA “as a legitimate response”.
“(IRA was) a legitimate response to British military occupation, to engage in armed actions, and thankfully we got to the situation where there is no rationale for that,” he told Belfast Crown Court on Monday.
When asked about the IRA’s policy of murdering informers, he said: “I accepted that if people - I don’t like the word tout by the way - that if people were agents or informers, and this goes for me as well as anyone else, they were liable to be shot.
“It’s regrettable, it’s not something I would advocate for.”
When pressed by a prosecution barrister, he added: “It is a regrettable fact that when armies are engaged in war, they do kill those they perceive to have assisted the enemy by giving them information that in any way jeopardise whatever, and that goes for all combatant groups.
“It’s regrettable, it happened in the 20s, it happened I presume at other times, I think the huge achievement of our time is that it no longer happens.”
However Mr Adams criticised the secret burial of IRA victims, adding he did not have a “carte blanche” for the IRA.
“I did not support all of those armed actions, I was very critical of a number of atrocities that occurred, sometimes accidentally, sometimes for other reasons,” he said.
“I don’t have a carte blanche support for the IRA.
“I also think, as we reflect back on what has occurred in my lifetime, and I am one of the people lucky to have survived it, but I always said those who condemned the IRA should come forward with an alternative, and that formed a main plank of Fr Alec Reid’s endeavours, and endeavours of John Hume and myself.
“Because those who were involved in denunciation, even when that denunciation may have been justifiable, did nothing to bring justice, did nothing to oppose the violence that was visited upon people in those working class communities or to prisoners in interrogation, or prisoners in prison.
“I am not a pacifist at one level, but at the other level, I think the huge mistake that was made in our time was when politicians talked to generals, and then it becomes militarised, and then it becomes what we have seen, and then these terrible incidents, including Jean McConville.