Magic was evil. Arthur knew that, and therefore, he didn't have a problem with disposing of those that practiced it. Usually, he only ever had to do it because he was attacked by some crazed sorcerer in the first place anyway, so it had never been much of a problem. It was self defense, nothing more.
But then he met the Druids. For the first time in his life, he had questioned whether all magic really had to be destroyed. It wasn't that he had talked to any of the inhabitants of the camps he had led his men to attack, and he knew his father believed them to be secretly plotting to destroy him, but in the end, Arthur had never found any evidence supporting such a theory. Their camps never held anything that would come in especially handy in war, and there had never been reports of Druids raiding a village or something else worthy of provoking any kind of action against them. All in all, the Druids – unless they were simply incredibly paranoid and masterminds at hiding anything that could possibly even hint towards ill intentions – were harmless.
And that was exactly what made Arthur hate them.
Every time he had to launch an attack on a Druid camp, he saw them run. They always ran. They never fought back, or at least not enough to seriously hurt anyone. They went with the least violence possible, even if it cost them their lives – which it did. And Arthur had to watch as his men slaughtered them by his own orders, hear the children cry and parents scream, older siblings sending the younger ones on their way while trying to buy them some time, despite how it almost always was in vain. He had ordered the women and children to be spared if possible, but he knew some of the knights would not listen. And he knew, as he joined in on the fight – if it could be called that – that innocent blood would be on his hands.
Arthur knew that despite their magic, the Druids never fought. And he hated what that made him.