Bloody tyrant or chivalric renaissance man?

If you read the letters of foreign ambassadors at the English court and contemporary chroniclers, Henry VIII was a talented, generous, learned, and pious king. At least at the beginning of his reign.

As a second son, he wasn't raised to be the king. Arthur was the heir, and Henry was the spare. Some historians assert that Henry was being groomed to enter the church, which made him a passionately religious man who fancied himself a theologian. The pope bestowed the title of Defender of the Faith (Fidei Defensor) for Henry's writings about the sacrements and ironically authority of the pope.

He spoke Latin and French, a bit of Spanish (probably because of his first wife), and tried his hand at Greek.

He was musical--he played several instruments, sang, and danced well. He also wrote music, though he probably did not write 'Greensleeves.' His most famous piece 'Passetyme with gude companye,' was played in the Scottish court and was a favorite of his daughter Elizabeth.

After his brother's death, Henry's father sheltered him from the gentlemanly skills his peers were practicing. Thus when he became king, Henry did them often and with zeal. He was a great rider, hunter, jouster, tennis player (which was a more brutal sport in its medeaval form), and archer.

Henry was well-read in classical literature and wanted to be known as a scholar. He was an amateur astronomer, cartographer, alchemist, and architect. He saw himself as a chivalrous knight, a brave soldier, a king after the Bible's King David. And he was for the first decades of his reign.

How did the Arthurian knight become a egotistical spoiled tyrant?

The seeds were planted by Anne Boleyn, and a man prone to vanity and used to flattery began to believe that he could not be wrong.

Indeed he staked his soul on this belief.