Nixon didn't know that the evidence tying him to the Chennault Affair was weak. If it was weak, he could play the "I didn't know" card. If it was strong, he would have had to play the much worse "It wasn't illegal" card. These are mutually exclusive, so conducting a criminal operation to get a peak at the evidence was a rational gamble. It didn't pay off, but no, just chilling was not a good option and no, the underlying act was not the lesser evil.