Trump in 2016:
She shouldn’t be allowed to run.
If she were to win this election, it would create an unprecedented constitutional crisis.
In that situation we could very well have a sitting president under felony indictment and ultimately a criminal trial.
It would grind the government to a halt.
Source: https://www.independent.co.uk/tv/news/donald-trump-trial-guilty-hillary-clinton-b2556563.html
Afaik the idea is that you want to avoid someone to be able to convict their opponents. So make it impossible to take them out using the legal system. Makes total sense to me.