The one and only thing I find interesting about Apocalypse as a character:
The X-Men and Magneto routinely argue about ideology and how best to end anti-mutant prejudice and prevent genocide. The X-Men argue that if Magneto is ever successful in subjugating all of humanity, then the tables will merely turn; mutants will become the oppressors and humans will become the oppressed, thus nothing would actually be solved by Magneto's tactics. Magneto counters these arguments by saying that mutants would never become the oppressors because mutants are inherently morally superior to humans (as bad as this argument is, it reflects arguments in the real world).
As far as I'm concerned, En Sabah Nur exists to make a liar out of Magneto so that we can watch him spew bad rationalizations in defense of his argument rather than admit that the most evil person embroiled in the humans vs mutant battle is a fellow mutant.
except that Apocalypse really isn't evil, any more than Magneto is evil - they're both 'bad' guys in terms of a narrative antagonist/protagonist perspective, but neither of them are exactly evil.
Apocalypse is basically a Chick Tract idea of what a Darwinist with superpowers would be, all the silly dramatic religious iconography included, except that he doesn't actually play out that way in his characterization. it's a crazy little turduckin of inconsistent writing.