Just ones I remember:
A not-so-cute "prank" on a gay student.
Blasé illegally impersonating police repeatedly.
The Seamus incident, for which he showed no regret or remorse later.
During the campaign, he would change his stance on topics day-to-day to fit the mood of the day's crowd. This was more than politician-speak. He had no guiding principles or beliefs. A good example was his Massachusetts health care law, which he tried to distance himself from to attack Obamacare.
While with Bain Capital, he bought and gutted companies rather ruthlessly, much like Al Dunlap as described in The Psychopath Test.
Your old boss was right and if you're interested in reading about this topic more I would highly recommend reading The Psychopath Test by Jon Ronson. There's a lot of fascinating info in this book about psychopaths and how they operate and there is a section that talks about the fact that there is a higher rate of psychopaths among CEO's compared to the general public. Unfortunately, in business it does tend to be beneficial to only care about yourself and what you can gain and to not have any qualms about screwing people over to benefit yourself and the company.