Shakespeare must have understood that the only way to make a character seem like a real human being was to give him (or her) contrasting traits. Even his Shylock in The Merchant of Venice is somewhat sympathetic because of the way he is abused by Christians. Even the fiendish Lady Macbeth is humanized in her sleepwalking scene as well as in the following soliloquy where she shows that she is capable of fear and that she loved her father.
Alack, I am afraid they have awaked
And ’tis not done. The attempt and not the deed
Confounds us. Hark! I laid their daggers ready;
He could not miss ‘em. Had he not resembled(15)
My father as he slept, I had done't.
Edmund is humanized when he expresses his feelings of resentment and humiliation in his soliloquy which opens the second scene of Act 1.
Thou, nature, art my goddess; to thy law
My services are bound. Wherefore should I
Stand in the plague of custom, and permit
The curiosity of nations to deprive me,
For that I am some twelve or fourteen moonshines(5)
Lag of a brother? Why bastard? wherefore base?
When my dimensions are as well compact,
My mind as generous, and my shape as true,
As honest madam's issue? Why brand they us
With base? with baseness? bastardy? base, base?(10)
Who, in the lusty stealth of nature, take
More composition and fierce quality
Than doth, within a dull, stale, tired bed,
Go to the creating a whole tribe of fops,
Got 'tween asleep and wake?
He not only feels personally outraged, since he had no say in his conception, but he sympathizes with all the men and women who are like him in being ostracized because they are “illegitimate.”
One of the standard techniques of professional writers is to create characters by giving them one dominant trait and one contrasting trait. This makes them seem real because all of us have good qualities and bad ones. Dr. Jekyll and Mr. Hyde is an extreme example. Both good and evil existed in one man, Dr. Jekyll, but he succeeded in separating himself from his evil side, at least temporarily, and developed a split personality.
King Lear himself has good and bad qualities. He shows himself as a stubborn, overbearing, selfish old man in the first act, but he becomes a pathetic character by the end. The only really "good" character in King Lear is Cordelia, but she is not totally believable. She is too good to be true.
Edmund’s character is very important because he is responsible directly or indirectly for much of what happens throughout the play. He is directly or indirectly responsible for the deaths of Lear, Cordelia, Goneril, Regan, and his own father the Earl of Gloucester. He is indirectly responsible for the death of Regan’s husband the Duke of Cornwell because he informed against his father, causing Cornwell to mutilate Gloucester and provoking one of the guards to attack Cornwell with his sword. So Shakespeare took pains to make Edmund seem like a real person with a legitimate grudge against his father and against society in general.