Opposites Attract

Question: How would you justify a character having purviews that are direct opposites to each other? Such as Justice and Chaos, Sun and Darkness, etc?

There are lots of ways to explain having seemingly opposing purviews; in fact, pretty much any reason a player can come up with is valid. Some characters may be proponents of balance (particularly those with the Harmony Virtue) and therefore find it prudent to have opposing purviews so they can use powers from either side to avoiding tipping the scales one way or the other. Others may be conflicted by what they want to do - maybe dear old dad wants them to follow in his footsteps as a Fire god, but what they really feel a passion for is Water, so they end up buying both and becoming a hybrid creature of both worlds. Still others (at least in our games) may end up with opposing purviews quite by accident - if one group of Fatebound mortals believes a Scion has Justice because he saved them from marauders and another group believes he has Chaos because he blew up a building in front of them, he might end up finding himself with positive Fatebonds to both. (This actually happened to Goze for a while, though he later ended up losing both purviews.)

Easy as it is to pick a purview and stick only to its concept ("I'm a god of the sun!"), there are loads of mythic ideas that are better expressed by a combination of purviews, even seemingly opposite ones. If you want to be God of Eclipses, you probably want both Sun and Darkness (maybe Moon, too!). If you want to be God of Natural Disasters, you probably want both Earth and Sky. If you want to be God of Life and Death, you probably want both Health and Death. There are tons of character concepts that easily encompass "opposite" purviews - honestly, I don't see any need for a character to justify having them. Wanting to have them and having a concept idea for them is good enough for me.

Keep in mind, too, that there are plenty of gods in various world mythologies with "opposing" purviews, too. Tlaloc has both Earth (as he is literally the foundation of the world) and Sky (as he is the thunderer and bringer of rain). Njord controls both Fire and Water, Baron Samedi and Nephthys are both strongly tied to both Death and Health, Set is notorious for embodying both Chaos and Guardian, and so on and so forth. Opposites aren't exclusive in Scion; in fact, they're a great place for budding Scions to find a unique niche for themselves that an older god isn't already occupying.