There are a dozen different ways Trump could have illustrated the 'pathological' application to Carson. Trump chose to go the child molester route.
Could you give me a few examples of conditions where people behave in extreme and unacceptable ways due to powerful feelings that can't be controlled that would be less offensive to you than child molester?
First of all, Carson was describing his temper as 'pathological', which by definition means that it is extreme, excessive, or markedly abnormal. Ben Carson revealed this as something that he had overcome in his life. It was a message of victory and hope for others who know someone dealing with this.
Donald Trump redefined 'pathological' to mean 'incurable' which is patently false. The whole point of Carson's revelation was to demonstrate that it is 'curable'.
As for the term 'pathological', the most common use is in describing an extreme, excessive, or markedly abnormal liar. Yet this did not fit the false narrative that Trump wanted to employ. He purposely brought up the child molester analogy in order to smear Carson, and then lied about it by saying that Carson had an incurable disease.
The irony here is that Trump himself is a pathological liar. Trump lies when telling the truth would actually be better. With Clinton and Obama, they lie about things that matter. Trump lies about things that don't. He simply can't help it.