In defense of Aatrox, i will look at Renekton:
https://vignette.wikia.nocookie.net/leagueoflegends/images/4/4d/Renekton_Update_Concept.png/revision/latest?cb=20170125062351
https://d1u5p3l4wpay3k.cloudfront.net/lolesports_gamepedia_en/b/b9/Renekton_Splash_0_Old.jpg?version=14661e71d2f47073f04c9df4d83d68ca
Aatrox, is not a "super spooky elite warrior guy", he is a former Ascended. The body that he has now its just a crude imitation of his former glory. And he wants to die.
All those things considered, it makes sense for him to have a human like body without armor, because the "armor" that he has, its not an acutal armor, it is some kind of "mirror" of his former Ascended form. If we compare Aatrox with Renekton, both have no armor on their chest, because they dont need protection since the Ascended warriors have an insane healing factor and a really tough skin that can not be pierced by normal weapons.
My point is that, Aatrox does not wear armor because his original form did not needed that and he wants to look like his original form.
... And since he kinda wants to die, it makes more sense for him to not wear protection.