It wasn't trauma.
If Gio could feel something like trauma or remorse after killing a living being at this point, it would be a miracle.
It was two other things instead.
Firstly, it was a memory of trauma.
That wasn't Gio's first time killing anything, but he'd only killed some weaker beasts at a distance. It was his first time experiencing the feeling of murder at such a close proximity, and more importantly, it was his first time killing a human.
He didn't heal from that trauma for a long time. He was forced to continue his mission. He had no other choice when his supervisor from WICKED was visiting him in the hospital.
Until several years later when WICKED was destroyed, Gio was never able to move on from that moment.
And even then, it was imprinted in his mind until the deaths of his loved ones made it insignificant.