Aster grew up seeing his brother becoming the hero.
Growing up in a family with high power in society's security, Aster saw how Valerian slowly became the lieutenant. The hero.
So the boy had thought that he would become like his brother someday. He used to admire Valerian so much that he would copy every little action his brother did and try to become a hero too.
The keyword here was 'used to'.
Aster grew up seeing his brother becoming the hero.
And he also grew up seeing himself becoming the villain.
Being in the hero's shadow wouldn't make you a hero too. You would not become the same person just by following that hero's footsteps.
Because someone already made that footstep first and received the spotlight. Therefore, no one would care for the next hero unless the hero himself retired or vanished from the world.
Aster was the shadow of Valerian's shadow. No one appreciated him unless he did more than his brother.