Well it’s blood vengeance, it’s not logical, and wouldn’t be necessary if Uther hadn’t executed a probably innocent person. He was just guilty of “sorcery” and there’s no indication he had brought harm to the kingdom before Uther beheaded him.
But Valiant wasn’t a magician, just a dick. I don’t know if we can count that because he probably would have found another way to cheat to kill Arthur.
Nimue was once Uther’s friend. It’s hard to say if she would have killed the entire kingdom if Uther hadn’t betrayed her and murdered all of her friends in very public burnings/beheadings. She should not have poisoned the water, but her actions were directly motivated by Uther.
But nobody knew what it was or how to defend against it, or kill it, because the weapon that could had been outlawed and literally purged from the kingdom.
Villager 1: Hey, what’s that then?
Villager 2: Looks like a Griffon. I’ll go get the old wizard, he’ll have it taken care of in no time.
Villager 1: Okay, I’ll alert the knights and their mage, too.
Villager 2: Wonderful! We just saved several lives!
Of course, that can’t happen because of Uther.
So if Uther hadn’t murdered his parents, he would have never had reason to attack Uther (or Morgana or Gaius). Alternately, we could have had the following scenario:
Gaius: Looks like magic ot me.
Uther: Quite
Gaius: Let’s have Merlin save her life.
Uther: Yeah okay.
But of course, it would be better for people to potentially die because magic is so evil.
Indeed. But like the griffon example, nobody was prepared for this sort of thing and they were entirely defenseless (if not for Merlin). If Merlin hadn’t been there breaking Uther’s law, Arthur would be dead again. So even though Uther clearly can’t stop all magical creatures, he’s actively hurting his kingdom.
But again it’s a situation that was 100% created by Uther and his policies. The Druids were never said to enemies of Camelot before Uther began his war against magic, and given the fact that both Nimue and Gaius were in Uther’s court, it stands to reason that the Druids were most likely either neutral or peaceful allies. Uther doesn’t arrest Druids based on whether or not they use magic, he arrests them for being Druids. I don’t see how that’s a morally defensible act.
But the Black Knight had a legitimate beef with Uther, as Uther sacrificed his sister for Arthur’s life.
Also remember why Uther has gone all bloody hungry for sorcerers. It’s not because they were laying siege on Camelot, killing people indiscriminately, or harming anybody. The only reason ever stated was that the vendetta began after Ygraine died. And his cover story was that “magic stole her” but I don’t believe for a second he didn’t know what he was doing. I can discuss that point more after you’ve seen the rest of the show.