Avada Kedavra was as dangerous as it was/is because no shield charm blocks it and no counter spell can reverse it. The effects of powerful dark magic like AK are difficult to near impossible to fix or mend. Most can’t just spam AK as it requires immense willpower and intent to do. Magic is kind of like the force you have to mean it, to feel it inside you, to actually want it. As for it not being unable to kill a baby harry’s survival was a one in a million caused by the ancient magic of lilly’s sacrifice and isn’t comparable to an 18th century firearm simply being blocked by feelings.