I believe that agreements such as the Non-Proliferation Treaty (NPT) and Comprehensive Nuclear-Test-Ban Treaty (CTBT) has helped make the world a safer place by helping most of the world have a framework within which we can research and use nuclear technology without having to have a global nuclear arms race.
I believe that with the current trends in technology and defense, and with the integration of cyber-enhanced services into the fabric of society, that governments, if not intelligence agencies, need to have a public agreement or understanding on what type of cyber offense will not be allowed.
This means that at least facilities whose failure can have a global impact (like the nuclear reactors at Three Mile Island) should not be attacked.