Aatrox's passive says bloodwell will fill 100% when damaging or taking damage from enemy champions or monsters. This however does not happen so I'm not sure if it's a tooltip error or a bug, either way it is quite misleading.
The key phrase here is "while hellbent." You have to already have 100% and be draining. Any damage dealt to or taken from monsters or champions before the blood well depletes will reset it.
It says while hellbent it will do what you wrote. Otherwise you have to build it up yourself. It's not misleading, you just didn't read it right.
