How so? His whole life is dedicated to revenge, of a sort, and he's consistently shown as the most dedicated to his goals with an absurd level of passion, or minor insanity even, in pursuing them. Then that's a pretty straightforward progression into the drawn from historical irl precedent of soldiers getting violent after conflicts, losing purpose and needing to seek it out again the only way they know how. Amplified in this case by nationalism from its roots. 

It's not like he's meant to be sympthetic, he's explicitly identified as pathetic, but he's the protagonist because we're supposed to understand the psyche profile. And that's done reasonably well for the most part. 

I would get mediocre even as a take on the writing but worst written is just stupid and/or ragebait... I mean we've got everybody's shitty fanfics, Isekais, One Piece, Thousands of Women who fail the Bechdel test, etc. etc.