One could say the same of atomic weapons, yet it's likely that nuclear weapons have saved more lives that we can even imagine. The impact of war was accelerating in raw numbers, and increasing in relative numbers, all the way up to WW2. 3-4% of the world's population was killed in WW2. In today's terms that would be ~270 million people. Think about that number for a minute. Let's put it in scales of 9/11s. During 9/11 2,996 people were killed. Our scaled WW2 had 90,120 times more deaths. That number is so absurdly large it's difficult to grasp. So put another way that's a 9/11 scale event every single day for 246 years. The United States, as a nation, was founded 242 years to give that number some scale. Just think about that! If each generation has children at 30 years old then another way of seeing that number would be a 9/11 event each and every day from today until 2 years after your great great great great great great great grandchild is born.

But then came the nuke. Nuclear weapons make traditional war between developed and nuclear capable nations basically impossible. Think about what the Cold War really was, or even what the geopolitical 'disagreements' of today are. Those are World Wars 3 and 4, averted because these wars would be unwinnable by any side. Like Einstein said, 'I'm not sure what World War 3 will be fought with, but the 4th will be fought with sticks and stones!'

Did the people developing nukes understand that they would finally create a weapon that immensely powerful that it would deter open unrestrained war for decades to come? It's possible, but I think that their motivation was something more straight forward - gain military superiority in the present, probably also mixed with a bit of scientific curiosity about the challenge of creating a nuclear bomb.

It's difficult to predict the future. The most awful and aweful weapon created in the history of mankind ended up creating the most unprecedented period of peace for mankind as well. This is also why I vehemently oppose nuclear disarmament. That's how you get WW3 if people actually disarm, though in reality it'd simply likely result in nations obfuscating their nuclear weapon programs and facilities. It's just a molten salt thorium reactor guys, come on - breeders are awesome!