In Nixon’s interview with David Frost, he says rather confidently when asked why he didn’t destroy the tape recordings, that he would have destroyed them had he thought they proved he did anything illegal. Keep in mind this is years after his resignation, pardon, and the smoking gun tape is known, yet Nixon is still able to claim he didn’t think the tapes showed him doing anything that would count as breaking the law. Is this just a simple case of Nixon lying to Frost, or being outright delusional? Or does Nixon have a legitimate legal point, that the tapes, while politically damaging, didn’t show him doing anything illegal? I’m confused about how Nixon could say this an interview, while at the same time, refusing to give the tapes over to prosecutors. What am I missing?
Nixon was probably lying to Frost. He subsequently gave other, conflicting rationales. In his memoirs, Nixon says that he did not destroy the tapes because he believed they would protect him against his own aides if they turned on him. He lays out several other reasons in his memoirs:
"Many factors bore on my decision not to destroy the tapes. When I listened to them ... I recognized that they were a mixed bag ... They indisputably disproved [John] Dean's basic charge that I had conspired with him in an obstruction of justice over an eight month period... I was also persuaded by Haig's reasoning that destruction of the tapes would create an indelible impression of guilt ... Finally I decided that the tapes were my best insurance against the unforeseeable future. I was prepared to believe that others, even people close to me, would turn against me just as Dean had done, and in that case the tapes would give me at least some protection."
Stephen Ambrose has a short article on the subject. One of his points is that Nixon did not believe the tapes would be released and thought that the Supreme Court would rule in his favor. Because, as Nixon says, the tapes were a "mixed bag", this made them incredibly valuable if deployed skillfully and Nixon was skillful. Absent the Supreme Court's ruling, he could have selectively released the exculpatory material and withheld the incriminating material. That could have worked out quite nicely for him because there were genuinely many exculpatory statements in the tapes as a body (Ambrose helpfully notes that Nixon knew he was being taped and likely said things deliberately for this reason at times). So it was absolutely the best of both worlds to keep the tapes but fight their release (as a body rather than selective Nixon-driven leaks).
As to what the tapes prove, I think it's fair to say that the "smoking gun" tape is not really a tape of Nixon committing a crime. It's only the "smoking gun" because of the way that it fits with the other evidence. It goes to the famous question of what the President knew and when he knew it, but it's not devastating in and of itself. Nixon would also, including famously with Frost, dispute the basic legal theory of the case against him so there's also some space between delusional and accurate for him to occupy.