First of all, they did bring this up in Thunderbolts.
Secondly, these comments glorifying Walker for killing that Flagsmasher "because they were a terrorist" are missing the entire point of US Agent's character.
Captain America's shield is meant to be used in DEFENSE of the innocent...not to PUNISH those who have wronged Captain America personally.
If John Walker was in Civil War, he would've taken Iron Man's head off at the end instead of having Steve's restraint.
John Walker is a mentally unstable jerk of a man who wants to be Captain America for the glory, respect and power.
What makes his character interesting and unique from Steve Rogers is that he is flawed and doesn't deserve to be Captain America...because Captain America protects everyone equally and has a good heart.
He murdered a guy in public. Like in broad daylight. Very graphically. Did they bring that up at all in Thunderbolts??