why is using nuclear weapons as a defensive mechanism bad?

Weapons are supposed to be a deterrent.

Nicole, because only a preemptive strike would be of any use.

Because by the time you use them it is too late? But that envisions one giant bomb like in Strangelove.

It is not for defense, it is a DETERRENT. Any use of nuclear weapons kills millions of people.

Were it not for nukes, the Soviets would have invaded American soil.

It has worked so far, but there is always the risk that somebody will use them.
They still seem necessary though.
A foolproof method of destroying a rocket before it detonates would be better. But foolproof is hard.

becaiuse it would ruin the world

It is very very good. It works.

What would you expect the retaliation to be??

As a method of preventing a nuclear exchange during the Cold War, we got away with it. But, sooner or later, an unstable dictator with access to these weapons (possibly Kim Jong-un) is going to find himself about to be deposed and decide to take the rest of the world down with him and attack his nuclear armed neighbors. The exchange is going to kill millions of innocent bystanders. Mutually Assured Destruction only works when both sides armed with nuclear weapons are rational.

99.999999% of humans have no choice it is the Roman Emperor lunacy Jack the rippers method of cleaning up prostitution in 19th century Britain or the lemming we all go but the bad guy goes too arm America to kill the shooter

Its not bad is it? MAD . mutual assured destruction is a defensiveness doctrine since it simply means that if a country which used nukes to attack another would l be destroyed in a retaliation ..

Having them as a deterrent is good for a country under threat or a superpower but they can never be used.

No idea.

They are not a defensive weapon. They are a measure of last resort and the only 'defence' they offer is MAD - Mutually assured destruction... ie you both convince the other side that using them would be a very bad thing for both of you.
There have been a lot of wars since the start of the nuclear age, involving nuclear powers - Korea, Vietnam, Falklands just to name a few of the more famous ones.
in none of them were Nuclear weapons ever used , because they are no use for defence and very risky to use as offence.
Any use of a nuclear weapon - even a **small** low yield device - would escalate very quickly

Mutually assured destruction