Pure spite.
That scar is a symbol. We no longer see Asami's scars, because they never held any meaning to Asami. However, Fei's scar to him holds meaning and are a constant remnder of that life-altering time of his life. The scar is not just present still on his body, but on his soul, his heart and his mind. His musing in Vol. 2 shows that very well. It's not "that time I was shot", but "the man who gave me that scar". Just as the scar remains, so does the past for Fei. It is a lingering echo of pain, of longing, of loss - not just with regard to his father but also with regard to losing someone who was never his to lose (Asami). That scar is iconic, because it symbolizes who the Fei is coming from, that we see seven years later. It is also still there to symbolize and underline that Fei has never had real closure. That's why sensei still features it whenever we see Fei. To show us where Fei is coming from when we see him seven years later.
Fei is quite literally a scared soul.
And by that I meant SCARed as in carrying a scar, not scared as in afraid. ;)
What you really meant was scarred, for future reference.
It was 3am when I wrote this and also, I'm not a native speaker. English isn't even my third language. For future reference.
Thank you, Anoni. That's what I meant. A symbol of his inner wounds.
Soooo why does Feilong still have a scar seven years along while after a few months, Asami doesn't, even though they were both shot in pretty much the same place?