This is because he was trying to conquer Europe over all, so he could make the "perfect" country, and have everything the way he wanted it, and in complete control over the entire situation.
I would also go with D.) Mutually assured destruction.
This whole idea mainly started during the Cold War. The United States and Soviet Union didn't want to use their nuclear weapons because they feared that if they did, the other side would retaliate. Ultimately, both sides would lose everything.
