As long as you are in the right,and did the right thing, it should'nt be an issue.
There's lots of cases each year of lawfuly-armed citizens intervening in other situations.
The greater concern is knowing who's friend or foe- if a situation isnt clear from the beginning (not observed from start,not obvious,etc. )
Say, if you observe a gun fight break out by 2 persons-but didnt see what caused or lead up to it,for example. How would you know who to engage? One could be a UC police, or even the "good guy" citizen.