Fritz Haber invented chlorine gas specifically for Germany in ww1. Clara Immerwahr, who had married him and was also a scientist, committed suicide as a result.
He also invented artifical fertilizer, which is responsible for the population boom of the 20th century. The jury is still out on that one.
In a similar vein, Project Pluto. Essentially a nuclear ramjet that could fly 150m off the ground at 3,700 km/h, was impossible to intercept at the time, could carry sixteen nuclear warheads and crop-dusted the earth with radiation everywhere it went. It was eventually cancelled for being “too provocative.” Which, coming from the US army, is quite a thing lol.
That assumes that you believe that the world would be a safer place if only one nation had nuclear weapons. I would imagine that would be the least safe of all possible scenarios.
If everyone has nuclear weapons at least there is the possibility they will never be used. If they are used it basically ensures the end of the world so, swings and roundabouts.
Your comment is only technically correct, so I am gonna add to that:
Alfred Nobel did invent dynamite and was also a believer in mutually assured destruction, BUT: those two facts are not directly connected.
Dynamite in itself was not intended for warfare, but for mining. It was still relatively unstable so not really suited for warfare. (TNT, which came around 1900, solved that problem.)
Nobel did invent smokeless powder for warfare and he transformed Bofors into an arms manufacturing company though.