Evil is in the eye of the beholder. Evil can mean cruel, unjust, wicked and various other synonyms.
What was the cause of Maeglin's actions? Jealousy, greed, anger. These things do not make a wicked person, but they may cause someone to do wicked things. Maeglin himself was not an evil person, but his actions caused him to do something evil. His thoughts were not malicious, and he merely acted out of lust and desire.
Or at least, that's the way I see it.
Uhmm....
In
Middle-earth,
Evil is most definitely not "in the Eye of the beholder."
Evil and Good (and Light and Dark) in Middle-earth are tangible, real things.
Manichean Dualism is true in Middle-earth.
This means that you can literally have a "handful of
Pure Evil." You can put it in your pocket. You can bake a cake with it. You can sprinkle it on toast.
The same is true of "Good," and "Light" and "Dark."
They had actual vats of "Light" that they gathered from the Two
Trees in
Valinorë.
Ungoliantë spun actual webs of "Darkness," and the fog created wasn't just so think that it blocked light, but rather it was an "Anti-Light." It was tangible "stuff" that was
Darkness Itself.
Tom Shippey discusses this in
The Road to Middle-earth, where he discusses the problematic nature of Manichean Dualism to a Catholic Theology, and to
Tolkien, yet for a pagan mythology, and for Anglo-Saxon and Norse Myth, Manichean Dualism was a very real thing within those myths.
In plain moral/ethical terms of Tolkien's Theological beliefs, Maeglin was not "Evil," he was Sinful. He was Covetous, and Lusted after Idril.
And, due to the Metaphysics of Middle-earth, this "corrupted" Maeglin (allowed "Evil" to work through him).
In
Morgoth's Ring (and
The Letters of JRRT) , Tolkien and CJRT (Christopher) both explain that Morgoth's rebellion caused his power to be diffused throughout the substance of Ëa and
Arda.
Literally the Universe itself was poisoned by Morgoth. Tolkien limited this poisoning to Arda, as the
Ainulindalë was really just specific to Arda, and not to all of Ëa, but simply Arda having been diffused with the Evil Substance of Morgoth is enough to cause any person to have that Evil corrupt them.
Remember that Corruption is a major Theme of Tolkien's within his work on Middle-earth.
Corruption is the major theme of the Abrahamic Faiths as well: That the Rebellion of Lucifer/Satan poisoned Creation. Although the degree to which this theme is present in any one sect within the Abrahamic Traditions varies not just between the sect, but within the person as well.
Given this, Maeglin was hampered by not having a sufficiently grounded Foundation because of the hatred of his father, Eol, for the Noldorin, and Mankind. This nurtured the seed of Evil within Maeglin, which may or may-not have taken root, and possibly planted other seeds of Evil as well.
And being a "Dark
Elf" (Moriquendi), Maeglin did not have the benefit of having lived in Valinorë (a
Holy Land), where he would have been exposed to the
Light of the Two Trees.
Having lived thus, most of the
Noldor were thus especially armored against Corruption by having been touched by the "Light of Creation" from the Two Trees (something not tainted by Morgoth). Not all Noldor were equally affected by that Light, and the
Sons of Feanor themselves did great damage to themselves with their
Oath. It caused that Light to fail within them, where others who nurtured it would have been more protected against Corruption.
So.... Magellan simply had been dealt a raw deal by both Fate and his own choices that led him down the wrong path.
MB