Over the years, some weapons have been banned by international agreement because they needlessly inflict too much damage or they're more likely to harm non-combatants. For example, chemical and biological weapons have been banned by international agreement and treaties. They're considered very cruel and, because military personnel are more likely to have defenses against them, these weapons actually present a bigger threat to non-combatants than to combatants.

Night vision goggles are not a banned weapon. Neither are anti-tank missiles and stuff like that. However, following Iraq's invasion of Kuwait in 1990, several UN Security Council resolutions were passed outlawing weapons sales to Iraq. It isn't that the weapons themselves are banned, just their sale to a specific country.