Disclaimer: I don't own Merlin.
Arthur was no fool. He was aware of what most people thought of his father. Many people saw him as a tyrant. They thought he was too focused on cleansing the world of magic. They thought his laws concerning sorcery were too harsh. Like most people, they focused on his father's flaws, not the man he truly was.
Arthur would not deny that his father had some serious flaws. However, he thought that that the good he did outweighed the bad. The man might have harsh laws, but his kingdom was one of the most prosperous kingdoms around. The people did not see that his father, in his own way, was trying to keep his people safe by eliminating something that he saw as a threat.
When the day came that his father was no longer living, Arthur knew that it would not matter how other people remembered him. What mattered was how he remembered his father.
This was based off of the quote "It doesn't matter who my father was; it matters who I remember he was." –Anne Sexton
