I think the series has some serious ambiguity for those who didn’t read the book! Who are John Uskglass and the Raven King? (In the series they said they are two different persons.) What do they want? Are they good men or not?
In the book, John Uskglass was the Raven King, a historical figure who, like Arthur, is expected to be capable of returning. He was a human child who was raised in Faerie, and whose magical powers exceeded all other humans. (For, as the book points out, Humans are weak in Magic but strong in Reason; Fairies are the opposite.) After re-emerging into the Human world, he gained a kingdom, presumably around Yorkshire (I believe Childermass comments about how the people there remember him).
At no point do I recall him being described as "evil" (as opposed to "good"), but he was powerful and inscrutable. He is remembered as a protector of his people.
I cannot find my copy of the book now, but the footnotes do contain a lot of what is known about the Raven King.
They're different names/titles for the same person.
John Uskglass is his human name - whether or not it's his real name, the one he was born with, is unknown, but there's nothing special about the name itself unless you know who it is. The title of "the Raven King" sounds much more impressive and mystical. Which name you use might indicate your stance towards him: "the Raven King" is a term of respect, whereas "John Uskglass" makes him sound more normal and mortal.
It's hardly feasible to write everything we know about him in the space of a Stack Exchange answer, but you can read more information about the character at the Strange & Norrell wiki.
As for whether he's good or bad, I don't think that question really makes sense. He transcends those concepts, like the fairies he grew up among. We might say the Gentleman with the Thistledown Hair is evil, but the truth is rather that he's neither good nor evil - he's just different, alien to us and our concept of morality. He's capricious, uncaring, and simply inhuman in the most literal sense. The same could be said of the Raven King. His acts might be seen as good or evil, but really he's neither.