But he is not an evil twin, or "bad kirk." Id/passion-Kirk is in fact good, for without him there is no Kirk-as-we-know-him. That twist, illustrating our id as a good and necessary component is a marvelous thing by Mr. Matheson. Along with the weakness and insufficiency of what we normally term "good." I show this ep in my psych class and have the kids cf. to Freud's tripartite model. I do not deny the episode's flaws, but because of its greatness, I give it a 10.
One of the great teleplays ever, imho.