I think that with Eöl (and a few other few characters) Tolkien came close to creating a 'grey' character. That is - he's not wholly evil, nor can I entirely excuse his behaviour. He obviously behaves irrationally in his hatred for the Noldor and a couple of other instances - but I don't think he is the kind of person that does evil for the sake of it - that is, he does not
act evilly, he
reacts 'evilly' or better said, 'rashly', turning this story into a tragedy... And in a tragedy everyone more or less is at fault and everyone has to suffer.
Not only Eöl is to blame for what happens in this story. Aredhel shouldn't have left her brother when he begged her not to, Turgon should have tried to win Eöl over and prove his kindness instead of saying "I will not debate with you', and Eöl himself should have been less impulsive, not to mention Maeglin, who was the truly disturbed one (a true First Age sociopath
)... But that's not something that could have been done, because these acts define the characters themselves: Aredhel left because she was independent and carefree, Turgon was really proud and scared of being unable to preserve his stronghold and position...and Eöl, as
Kuruharan said was driven to it by the loss of everything he ever had (good comparison with Denethor, another 'grey' character.)