The description of the cloudkill spell says (emphasis mine):
You create a 20-foot-radius sphere of poisonous, yellow‑green fog centered on a point you choose within range. The fog spreads around corners. It lasts for the duration or until strong wind disperses the fog, ending the spell. Its area is heavily obscured.
The fog moves 10 feet away from you at the start of each of your turns, rolling along the surface of the ground. The vapors, being heavier than air, sink to the lowest level of the land, even pouring down openings.
If you center the cloudkill spell on yourself, does the cloud expand outward on subsequent turns, since it moves "away from you"? Or would it stay as a single unit and roll off in a direction of your choice?