In a way, John is like Blonsky from The Incredible Hulk. Didn't start off as a monster, but by trying to meet expectations and their own desire to be "the best" drove them on a negative character arc. Where they feel the need to go through any means necessary to prove to themselves and others that they are more than the myth they tried to present. Hell, Ross calls out Blonsky on why he would still do tours when he is in his late 40's. Blonsky's goal in that film was to prove that he was the best. He asks Ross if Banner was a fighter the moment he got his orders. Getting rekted by the Hulk only pushed him towards those extreme ends. Taking the serum that enhances positive or negative traits of a person didn't help either of them. Walker is trying to live up to Steve's legacy. Which is why he tried so hard to gain the approval of Sam and Bucky in episode 2. He feels that without their approval he can never truly be "Captain America".
Honestly I am eager to see how Marvel will handle Blonsky in the She-hulk series they have in production. Was a surprise they got Tim Roth back for the role too.