the term Maverick could have been invented for guys like Trump. like you say watch and be afraid...he doesn't seem to get on with anybody very much.

That's not the American understanding of the term maverick. If someone is "a maverick" they're independent, it's meant to praise their ability to function apart from the herd. It doesn't mean scary or can't get along with everyone else.

I think the term you're searching for might be "grouch" or "outlyer."