Interesting! Sorry it took me so long to reply. So here's my take on Eöl:



Eöl is evil, or at least as close to it as an elf can be. But he's evil with streaks of goodness, and evil in a human way (as opposed to Morgoth or Sauron, who are what I'd call "cosmically" evil). While there may be numerous possible explanations for it (family murdered by the Noldor, dark/antisocial nature, etc.) He's someone who commits wicked acts, but his motivations are logical. His love for Aredhel is not enough for him to overcome his hatred of her kinspeople, and so, consumed by jealousy and wrath, he kills her. Maeglin, on the other hand, is at first resistant to his own nature. He fights alongside his uncle in the Nirnaeth Arnoediad, showing his loyalty is to the Noldor, the very people who killed his father (and his ancestors). And when he is captured by Morgoth, he doesn't immediately think, "Hurrah! This is my chance to take over the world!" He resists - only after he is tortured and manipulated does he give in. Even though ultimately he, like his father, is unable to overcome his wicked nature, he tries as hard as he can to do so.