I'm still not sure why only the flag area only instant kills the other players when a gang took it and not outside of it. Not fun when everyone can just rush out and wait then come back in.

Simply saying, only gang who took the base will instant kill people IN the flag room area where people are trying to take the 'safe'. And they can't get annihilate outside just right out of the flag room. (Door which leads you into flag room)