Why is God’s name did he go on O’Reilly to begin with? It completely steps on his own anti-Fox message.
Answer: Because Donald Trump is a man of principal who keeps his word and his promises, in spite of what it may personally cost him; that's why. Mr. Trump had personally given his word to Ted Baxter's Bill that he would appear on his show for an "exclusive interview". He had given his word to appear long before Fau News's childish, utterly demeaning, personal attack/hit piece press release, that directly led to Trump's firm decision to skip the next Fau News "debate".
Having already given his word Mr. Trump was faced with a moral decision: Do I keep my word, inspite of how shabily I have been treated, and the ridicule I will undoubtedly face for my decision, or do I break my word using the reasonable justification that due to current circumstances I am unable to attend?
Donald Trump had no alternative but to attend the interview because his word is his bond, and unlike lying O'Reilly; Trump keeps his word.