It sounds to me like a vote-purity conscience snag on his part. Nobody needs to keep a promise to do something evil.
This could be solved by Kasich agreeing that he disagrees with where Trump is about some issues, while also conceding that Trump is the best practical route to progress towards the good for America. And please do not take his political support for Trump as an endorse-all for Trump’s current agenda, and that he hopes Trump will remain open to course corrections as he goes.
Evil, you say. You should both make your case that a vote for Trump is a vote for evil.
Until then, I'll conclude they were writing checks with their mouths they couldn't cash. As a politician, words and commitments are the only currency that exists. Why would anyone vote for a politician who can't keep a commitment?