Duty and Honor
People seemed to think it was easy for him to send wizards to die, but people often have the wrong idea about how a King really thinks and feels.
There was no pleasure in the killing of people he had grown up being taught were innocent and simple. There was no pleasure in his crusade against magic, but there was no way out of it. He was honor-bound to banish all magic from Camelot.
Younger people didn't really know how it was, before he conquered the throne. They knew nothing about his usurper uncle, and how he had tricked his father, with magic, to disinherit Uther while he was still a boy and send him to live with the common nobles. They couldn't remember his reign of terror, or how his friends used dark magic to control the kingdom, peasants and nobles alike.
They couldn't really understand why he despised magic, but they had never been treated as he has been treated, like a shameful thing that ought to be hidden away. They hadn't seen their own father become a puppet in the hands of a master magician.
But even then, that wasn't the only thing that turned him against magic. Still young and naïve, he had believed some of those people were good in their hearts, that they could use magic for good. He had learned the hard way that magic used them, not the other way around, and if allowed to have its way, it would destroy everything in its wish to control the lives of human beings.
He had trusted Nimueh, and asked her to give him a child, not knowing he would kill his wife in the process. Ygraine was still just a young woman, not yet twenty summers when she finally got pregnant - but Uther was almost thirty, almost a middle aged man, and he needed an heir. He had waited four years, and there was no time to lose. So, when he finally saw that he could father children, he had asked both physicians and magicians to find out why Ygraine hadn't conceived yet.
When they told him she couldn't, he wouldn't accept it - his guilt eating away his soul, his duty to the realm dominating his life, and he was ready to give up everything to guarantee that she would have everything she had the right to: a loving husband, a secure crown and a healthy child. Even if it meant using magic.
He never even dreamed that it was her life that would be claimed, that he would lose her without giving her everything he had promised.
So his honor demanded that he fought against the evil practices that had stolen his wife's life, and while he could bring himself to trust Vivianne, whom he loved, and Gaius, who stood by his side from the beginning, he couldn't allow anyone else to use magic without being punished. If even he, who was the crowned and anointed king couldn't use it without paying a heavy price, he would guarantee that the same was true to his subjects.
And, some nights, he would still hear their screams, he could still see their faces, mothers and daughters, fathers and sons, old and young, claiming for mercy, but magic had shown him that it didn't know mercy, and he wouldn't wield.
He did what he had to do, to guarantee that no one else would go through what he went through; to keep his kingdom from falling into the chaos that came from lies, frauds and tricks. He did what he had to as he was bound to do. And if it kept him awake at night, no one else needed to know, or seemed to care.
No King's decision was easy, but he did what he had to do.
It was his duty.