When I started reading his story, I felt a great deal of pity for the fellow and felt he had a lot of potential. I'm doing all this from memory so correct me if I am wrong:
Product of a spirited
elf woman basically kidnapped and raped by a tyranical monster. Bad start.
Longed to produce things such as his mother described in
Gondolin. Leaning towards good.
Saw his father kill his mother and then watched in silence as the
elves executed his father who died cursing him. Really bad start.
Fell in love with cousin. Strange. Cousin saw something dark in him. Bad sign. Cousin really can't be blamed in rejecting him, but it doesn't help the situation at all.
Cousin finds human lover.
Now I see this as a turning point where Maeglin can either get over it and pursue other interests (such as his craftmanship) or sink into bitterness. Apparently he picked bitterness. Downhill from there.
So he was a mixed bag who chose the wrong path about midway through the story . .. with a lot of help from situations beyond his control. Because there are tons of characters in
Tolkien who defeated worse to become heroes, I can't really coddle the character (oh, poor victim, couldn't help himself.), but I think he deserves some pity for what he went through.
Always get an early intervention desire when I read about characters like this.
If you want to take a character I find an interesting paralel from another book, there is the man from The Tale of Two
Cities. Reminded me a lot of Maeglin in some ways, both had promise and had fallen in love with a woman who later fell in love with a man other than him. Sidney frittered his promise on drink. Maeglin frittered his on bitterness. When it came down to it and both were asked to make a sacrifice Maeglin refused to endure pain to save his country, Sidney agreed to lose his life to save his beloved's husband.
Every good character starts out with a mixed bag and is given a point where they can chose to be one thing or another. Maeglin definitely had a more distinct chosing point, but I think that's all there is to it.