The most obvious answer is the sheer magnitude of his narcissism. Trump is very unpopular among young adults. Most of them believe that Trump is immoral and sexist. Many people also think that Trump is a racist. Trump made some remarks in his public appearances that were interpreted as racist remarks.
Many people also consider Trump to be arrogant, and they think that he has a massive ego. Trump has also criticized and humiliated people in public. Obama fans dislike Trump. Trump criticized Obama many times during his presidential elections, and he claimed that Obama was lying about his place of birth.
It will be hard to judge someone based on the type of image that he shows people. There are some people like Trump whom people believe to be mean and incompetent but no one knows exactly what he is going through. Who knows if the persona he is showing is something he believes will work to his advantage?
All people have something good inside. A bad person is someone who has no intention aside from his own interests. The actions that a person does may reflect who he is as a person but is that person necessarily bad? This cannot be answered immediately. This question is very subjective so people will always have different answers.