What if you know that 80% of the ppl who don't belong to a gang directly support it and as a cherry on top want to kill all people of the other territory. Let's say on both sides.

The question now is: how or if at all should the outsiders protect those 40%? 

 Probably not & if they are worthwhile they'll be willing, able & intelligent enough to figure out a way to defend themselves, preferably in an unconventional manner. 

 Wanting to & being willing to / capable enough by themselves are two different things. 

 They let that "gang" usurp that function that they should have been doing themselves for a reason...