I use this as a general guide for ability leveling, but one of Aatrox's best features is that his skills should be leveled up based on your needs. Going 1v2, or playing someone with a hard poke? Level Blades of Torment(E) up before anything else. Getting beat in melee exchanges and need more sustain? Level up his Blood Thirst/ Blood Price (W). Dominating your lane and want to zone your opponent to death? Level up Dark Flight (Q) and keep him away from CS. Don't just level up skills blindly with Aatrox, build him to fit your situation.
Blood Well (Passive)
Aatrox stores a portion of his abilities’ health cost in the Blood Well, which slowly drains to a base amount while he is out of combat. When Aatrox loses all of his health, he draws on the Blood Well and recovers the amount of health stored.
Dark Flight (Q)
Aatrox rises into the air and slams into a target location. Enemies within the impact zone are knocked up and take damage, while nearby enemies take damage.
Blood Thirst (W)
Aatrox heals himself on every third attack. Activating Blood Thirst toggles Blood Price.
Blood Price: Every third attack now deals bonus damage instead of healing Aatrox. Aatrox loses health based on the bonus damage dealt.
Blades of Torment (E)
Aatrox fires a line attack that damages and slows enemies caught in its path. Aatrox is refunded a part of the health cost if Blades of Torment strikes an enemy champion.
Upon activation, Aatrox instantly damages enemies in an area around him. For the duration of Massacre, Aatrox gains bonus attack speed and range. Massacre has no health cost.