Hi ! First message here.
If a global disaster would happen on earth ( And I somehow don't wish it ) Would the value increase or decrease ?
I'm not talking about solar flares or EMP. But more like big wars or great natural disaster.
What are your opinions on the subject ?

It depends on the type of disaster. A disaster affecting the electrical grid would hamper transactions and cause the price to crash. A natural disaster such as a hurricane and typhoon would be localized, so it wouldn't have an effect given the global nature of bitcoin. A world war might cause people to flee to precious metals that are physical perhaps.
What I believe to be especially beneficial to bitcoin is a man-made type of disaster: hyperinflation.