Assume that the president is caught red-handed in flagrant violation. Dealing with him "criminally" takes two steps. Congress impeaches and convicts him, but the remedy that Congress can impose is limited to throwing out of office and disqualifying from office. After that (and technically, even before that, but no prosecutor is going to charge a sitting president with a crime), he can be charged with a crime and tried for that. The penalty can include capital punishment.
Same sequence would come into play if a president commits murder.
Take away his secret service detail and see what happens.