The high priest and others viewed it as blasphemy because they were looking for ways to trap him (Jesus). Also to compare youself to the king was considered wrong or tho say that you were God's son was wrong.

The high priest just did not believe that Jesus the Son of God was actually human. He as well as others, apparently expected God "supernaturality" to show on his face. They did not see or hear that he descended from heaven and had a radiant light shining around him, hence, He could not be the Son of God.  Also, the fact that Jesus was "lowly and humble" sanctioned their belief. From the question, "can anything good come out of Egypt?" it is evident that the high priest answered the question negatively!