Now that I’ve drawn both Apex and Ruthnaan’s ref, I can go into more detail about them and their rivalry. The reason that Apex is evil isn’t only because he’s all the bad/dark attributes of Ruthnaan, or because he was given the purpose of Destroyer. It is because of a deep rooted jealously and hatred.

Apex is envious of Ruthnaan’s creations/creator abilities. While his are seen as beautiful and pure, Apex’s are corrupted and no matter how hard he tries, nothing he makes comes even a little close to how loved and adored Ruthnaan and his creations are. So, he gave up trying to make things for others and it was a wake up call to realize that his purpose wasn’t to create, but to destroy.
In fact, it is Apex that made the Seven Deadly Sins a thing, simply because of his own feelings towards Ruthnaan. If he couldn’t be loved, then he would be feared. This is why he spends so much time with demons. They are his most prized creations and the closest thing to him as an entity. He does with them as he pleases. Including taking away his brother’s own creations, case in point, Reapers.

It has been a constant battle since the dawn of time for them, and nothing has ever been able to get in the way of the universal war between good and evil.

(Side note, not on their lore, but a fun fact as to how what they do has subconsciously shaped their tendencies. Neither of them need to blink, since their bodies simply don’t require that function. Ruthnaan actually does, only because he has found that blinking makes others more comfortable than a non-blinking stare. Apex, however, doesn’t blink and even if he were to, he would blink his eyes in order from top to bottom instead of at the same time like Ruthnaan. Which, he has also found is better than one at a time for mortals.)