To quote myself from an earlier thread:
Being greedy does not equate to evil. It's a fast track to it yes but that still doesn't make MX less evil just because he isn't pure darkness or Vanitas evil by default because he is.
Darkness feeds on negativity not evil itself. Shown by even beings of pure light like Cinderella who expressed emotions of sadness and maybe even depression drawing the embodiments of negativity, Unversed, right to her.
And if a being lacking any darkness can show negativity that means a being lacking light should be able to show the opposite.
Point being Vanitas was created from Ventus by MX, trained by MX, all his time was spent either with MX, fighting Ven or Aqua.
Vanitas didn't have a chance to show anything otherwise than the evil he was exposed to by Xehanort.
And look at the list of things done by Vanitas compared to Xehanort. Can you honestly still say Vanitas is more evil than MX simply because of what he is?
No. It's the circumstances of his life that make him how he is in conjunction with darkness' appetite to feed on negativity.
Not that I disagree that MX isn't pure evil although I find it highly impossible any good still remains in him at this point, but my point is that just cause he's not pure dark doesn't equate that he's not pure evil.
This is also shown in the fact that everything is a mix of light and dark with the exception of 9 people, only 7 of which is natural.
The only possible way MX could ever be pure darkness is if he ripped the light out of himself the same way he took the darkness from Ventus because that's the only other way for pure hearted beings to exist in the KH universe.
So no just cause MX isn't pure darkness doesn't mean he is pure or next to pure evil.
Darkness is tied to the negativity but I think we've already seen plenty of reasons, in cases listed as well as with Riku and Eraqus, why it's not as simple as "darkness equals evil" or dictates the level of that evil.
It's far more complex than that.
EDIT:
The best example of what I mean is Terra. He's far from evil but still has a strong affinity for darkness and has since before MX meet him. In fact it was his affinity to darkness that gave MX the idea to target him to start with.
Nothing Terra did in the story was evil and what he was tricked into doing was unknowing on his part so he had no evil intent.
The only time Terra expressed something to feed his darkness was the hatred he developed for Xehanort which finally set his darkness loose but even then Terra didn't lose himself, just made himself open to MX's possession.
The MX and vanitas parts aside, I definitely agree and I think it's fairly easy to see that within the KHverse good, evil, light & dark aren't as simple as we view them here.
Terra is far from evil yet holds strong affinity for the darkness for no explainable reason other than he wishes to be strong. A desire that in of itself is neither negative nor evil.
Riku was able to channel darkness for good after he got some sense knocked back into him.
Eraqus' misguided views almost caused Aqua to attack the evil Stepmother and her daughters.
Eraqus attacked his own students in an irrational way when he'd have been a far better help if he had stayed calm.
I think Master Xehanort describes it best when he tells Terra that "darkness can't be destroyed, only channeled".
He words if as if darkness as an energy to be manipulated rather than an evil sentient force.
Darkness if like fire, both wish to consume and spread but neither are evil themselves just following their nature.
Light is the opposite to darkness' nature but if you have to much light you only blind yourself with it.
It's definitely nowhere near as simple as light=good, dark=evil as in most series. It's all dependent on the person themselves.
Like that famous saying goes "for there is nothing either good or bad, but thinking makes it so."