President Gerald Ford, pardoned Nixon after his near-impeachment. (Nixon resigned first. The pardon rendered impeachment and investigation moot.)
President George H. W. Bush pardoned six alleged traitors involved in “Iran-Contra” in the Reagan administration, smuggling weapons to Iran which was then an enemy of the USA, to secure the release of hostages and to illegally support a revolution in Nicaragua.
President William J. Clinton was impeached for concealing an affair. This became a continuing stain on the Democratic leadership.
In 2006 Democratic House Speaker Nancy Pelosi said that the “Impeachment [of George W. Bush] is off the table,” despite the Clinton precedent and the great crimes of W. Bush.
President Obama declined to prosecute any of the Bush administration criminals, despite strong cases against them. He also declined to prosecute all but one of the bankers who caused the crash of 2008.
These people who committed crimes were left free to rehabilitate their careers. Nixon’s advisor Roger Stone participated in Trump’s Presidential campaign. He is under investigation by Robert Mueller and is alleged to have been one of the Russian connections to the Trump campaign.
We let the scorpions sting, then we let them free to sting again.
I do not know when, if ever, the USA will return to a just legal system, but part of that return has to be trial and punishment of these criminals. Laxness in punishment lets them rehabilitate their careers and commit yet more crimes. Punishment, even for minor crimes, damages their careers.