Uther is…not the nicest guy you're ever going to meet. He's not a villain, exactly, but that's kind of not for lack of trying.
Let's see what makes him tick.
Merlin meets Uncle Uther in the least favorable circumstances. He's had to steal Uther's cloak (okay, some food, too) to keep from dying—and Uther's having none of this:
He was tall, young, with light brown hair showing reddish in the torchlight, and an elegant beard fringing his chin. His eyes were blue, and looked angry. He was cloakless in the cold. He had a whip in his left hand. (II.4.4)
Just in case you're wondering, Uther would have zero problem using that whip on a freezing and desperate young boy. He's only stopped from beating Merlin to death by the arrival of his much kinder brother, Ambrosius.
Things don't get much better between Uther and young Merlin. At first, Uther hates him because he thinks that Merlin is his brother's "catamite"—that's, um, a boy sex slave. Uther is, like, 100% wrong about that, but he can't guess at the true relationship between Ambrosius and the new kid on the block until it becomes clear how alike Ambrosius and Merlin are…
…Because Merlin is totally Ambrosius' son.
Yeah, that produces another source of friction: Merlin is a kink in the line of succession for Uther. It's lucky for Uther—and Merlin, really—that his nephew has no ambitions for the throne of Britain:
"I don't see myself as a king. Half a king, perhaps, or more likely a quarter. […] Perhaps Uther and I between us might make one, if you go?" (II.12.80)
Merlin's playing here, but he's about to learn that Uther's not, especially about his kingdom. Whatever Ambrosius wants or intends for his brother and son, it ain't gonna happen.
Before we go any further, we feel we ought to make one important concession: Uther's a really good soldier. It's just that he has no discipline, which is a major problem for the next guy in line for the British throne to have.
Unlike the highly controlled and thoughtful Ambrosius, Uther has a serious sweet tooth for the ladies. It doesn't really much matter which lady, just as long as she's beautiful and available. And, actually, "available" isn't even really a requirement.
Uther's reputation is, well, legendary.
Look, in a 5th-century army barracks, this isn't shocking stuff. It's not even a problem. But there is one particular girl who adds to the hostility between Uther and Merlin: Keridwen. She's the fine girl who keeps the gate at the nunnery—until she puts the moves on Merlin in the meadow above his cave. Merlin is really into her, but things don't work out. Let's just say that Uther picks up where Merlin left off.
Merlin can handle all of Uther's garbage—and there's a lot of it—but this is nearly unforgivable. Nearly. When they're about to part ways at Stonehenge, Uther extends an awkward olive branch:
"Do you hold the past against me, Merlin the bastard?"
"I hold nothing against you, my lord."
"A girl in Caerleon. You could call her nothing."
I saw him stare, then smile. "Which time?" (V.2.37-41)
Yeah, this is not nailing the art of the apology, but we'll give him, like, a participation award for at least showing up. Uther's lack of sensitivity makes it sooo much easier to dislike him all the way to the end of the story.
But wait—Uther's sexcapades don't end there. He gets himself into major hot water with Ygraine, the beautiful wife of his BFF Gorlois. Gorlois, the Duke of Cornwall, is old and not so hot anymore, but he does keep Uther from getting beaten to smithereens in his first battle as king:
[…] for all Uther's brilliant reputation as a solder, he was untried as supreme commander, and it was already known that he had not his brother's calmness and judgement in the face of odds. What he lacked in wisdom, he made up in bravery; but even that would not defeat the odds that came against him that day at York. (V.1.2)
Even though Uther owes his stupid life to Gorlois, he still finds it impossible to keep his hands out of the cookie jar. He gets Merlin to help him into Ygraine's bed in the most improbable and disastrous way. Merlin, of course, only helps him out because he has bigger fish to fry: Uther's about to become the daddy of the greatest king that will ever, ever live.
Merlin has his head on pretty straight about his status in the world: he's merely a "reed" for the god to blow through, just an agent of Fate. But Uther has no humility at all. He thinks he's all that and a plate of fish 'n' chips.
And for the most part, Uther is right to be sure of himself. Power totally suits him. Even Merlin notices this after their victory against Vortigern: "Now, like a hawk being flown for the first time at the quarry, he was feeling his power. I could feel it, too: it clothed him like folded wings" (IV.5.82).
But it's one thing to feel comfy in a power suit and another to be a man of power. Merlin tells us that Uther needs to hold up: he's merely a cog in the great big wheel of fate. After their disastrous night at Tintagel, Merlin tells a dying Cadal how it really is:
"Do you think Uther is a king, Cadal? He's but a regent for him who went before and for him who comes after, the past and future King. And tonight, he is even less than that: he is a tool, and she's a vessel…" (V.6.62)
Neither the gorgeous Ygraine nor the prideful Uther is worth as much as the little seedling of a King Arthur they just made. For all his skill and despite his fabulous beard, Uther is just the right guy at the right time.