Can you cite an international agreement that says that use of nuclear weapon for defense against an aggressor is considered as war crime?

> Where do you?

In the world where RF is stupid enough to invade the nuclear country.

I'm from Ukraine.

Using a nuclear weapon against another country with nuclear weapons is likely to result in the destruction of most of Europe at least. All military planners involved know this and nobody wants it, which is why this scenario is very unlikely.

Ok, so Europe should put more effort into taming RF and protect Ukraine. As I see, Germany, at least, doing the opposite, so it looks like EU ignores this scenario.

Europe, Russia and the US should all de-escalate enough to avoid a devastating nuclear war, yes.

To be fair, the only one even considering using nuclear weapons seems to be you, here on HN. No military or political planners are considering it because nobody wants to see Europe devastated in a nuclear war, and everyone understands this, which is why -- if there is war, hopefully not! -- it won't be nuclear.

If you think about it, YOU, an Ukrainian, don't really want Ukraine to attack Russia with nukes. Why? Because then you'd be dead, plain and simple.