Nuclear weapons and the advent of mutually assured destruction has ushered in a world where many, many, many fewer people die of violent conflict.
Technologies that cause destruction are not congruent with destruction. They change the game such that destruction may actually be prevented by their existence. They are tools, nothing more.
It's possible they have permanently eradicated world war. If that's true, they are not fundamentally bad but in fact really quite great. Only time will tell.
The possibility that nuclear deterrence has helped the world to be more peaceful in comparison to the past means that it's not such an easy answer. Hasn't even been a century since nuclear weapons were invented though, so no clear statistics.
