But the point is that they aren't 'well-grounded' fears. They're just prejudices based on a personal distaste for him. You say yourself that he hasn't actually 'done' anything wrong.
To my mind, Trump has to reassure his regional allies that the US will act if they come under real threat and that he isn't going to be a massive pussy like Obama was with his ever-disappearing 'lines in the sand'. A bit of intemperate language was definitely merited.