I disagree. In my opinion, Ishamael would argue that any distinction made between Good and Evil is ultimately meaningless. And therefore he has no interest at all in trying to define such a distinction.

Ishamael is a Philosopher, and his specific area of study in the AoL was "meaning" - particularly, the meaning of life. And, in the course of pursuing that study, he convinced himself (through what seemed to him to be a logical train of thought) that all acts & endeavors - essentially, all existance - is futile and meaningless. That's what Rand was referring to when he told Ishamael in TGS: "You were always so full of thoughts, Elan. Your logic destroyed you, didn't it?"

Look at the books Ishamael was purported to have written in the AoL (before he turned to the Shadow). I don't have the Guide with me, so I can't quote the book names exactly, but they were all essentially about the meaninglessness of life. The weren't about Good and Evil at all.

Ishamael is the very epitome of a nihilist. And that nihilism makes him want to destroy the Wheel, and therefore destroy the cycle of birth, futility, meaninglessness, and death it brings (as he sees it).

I don't believe Ishamael turned to the Shadow because he has any special regard for the DO. He turned to the Shadow because his goals coincide with the DO's. Siding with the DO gives him the best opportunity to fulfill his destiny, as he sees it. And, he tries to lure others to the Shadow for the same reason.