In short,
The King wanted More executed because More couldn't betray his religious beliefs and conscience to approve of the King's divorce to marry his mistress.
The King then wanted him executed because he wanted to make an example of him and because he knew how the society respected More and his beliefs so if he couldn't have More suppotring him he didn't want More at all.
They charged him on purgery, despite the fact that More never actually said he disapproved of the marriage, he just never agreed with it.
That's a very basic view, I haven't thought of this since my English exam.
I hated this book to begin with but once I read about the history and looked into the themes it began my favourite text for the year.
Good luck