There's a section of the game where Batman does have to kill a badguy to advance further
by using the batarang to cut down one of Joker's guys hanging from the ceiling in the Medical Facility, causing him to fall through the ceiling, which then allows access to the controls for the vents to clear gas from the room
I get the feeling a GTA sized map would require a lot of disk space and there might need to be a compromise on quality and depth. The reason I loved AA so much was the attention and atmosphere across the whole environment. Attention was paid to almost every hallway, which is not something they could do with a larger map.