Ok first off, if every country would have nuclear weapons and all, that would be the worst thing that would ever happen in the history of mankind.
1. Any country may use it for total domination, which is why it's a bad idea. We can't trust one another.
2. There are countries who actually want to eradicate all life in one country example: Israel vs. it's enemies.
3. A few countries have corrupt governments and possibly terrorists that would try and take the weapons to cause conflicts between countries.
4. The worst scenario is many innocwnt people would die if any nuclear war would happen.