Does anyone seriously think Donald Trump is afraid of Megyn Kelly? Ted Cruz has debated, in effect, 7 times before the Supreme Court yet if the Donald is afraid of him it doesn't show. His decision to skip the Fox News debate is not personal, it's a tactic, and time will tell if he overplayed a very strong hand. There are two possible ways Trump stands to win from his withdrawal from the Fox News debate.
Trump may just be trying to boost his earned media, a term never heard before he appeared on the political scene. It's a journalistic euphemism for free publicity and was coined by the various media that have been duped into providing him free publicity. Somehow in its convoluted way of thinking the media thinks we will feel better toward them if they can affect the image of an all knowing, fair and balanced arbiter of media attention which can dole out bits of attention to those who justly earn it. As you can view Trump's earned media as measured by Google Trends took a quantum leap when it became apparent he would not be appearing in the debate. If you are reading this at a later date remember the graph is updated in real time and may not reflect that surge of interest.
The other possibility would do Mario Puzo proud. Suppose Trump made Roger Ailes an offer he couldn't accept. Get rid of Megyn or else. Trump's campaign has been a high wire act from the get go and this tactic could succeed or fail in grand fashion. He has the advantage of watching the reactions and commentary for the next day and a half or so and if things are not going his way announce a change of heart. That would cost him some credibility but prevent an all out disaster in Iowa. Suppose though, that opinion is mixed or in his favor by crunch time? He can skip the debate. It could be that Trump senses he has a strong lead in Iowa. Then appearing in the debate only aids Ted Cruz and the rest of the field. Two quick wins in Iowa and New Hampshire maybe all Trump needs to secure the nomination as the rest of the demoralized field frets, moans and goes home. Whatever Trump's motivations may be Kelly is little more than a prop. "Nothing personal, Megyn, it's only business".