It has to be on purpose. It can be by a terrorist or state. If no humans die because the weapon misses or because it was aimed at infrastructure or meant to pollute the atmosphere or similar it counts.

Only a very extreme case wouldn't count as offensive. E.g. if an employee at a nuclear test site broke in and overrode the orders and blew up the bomb underground and no one died it would not count for this market. If someone stole a nuclear bomb and then blew it up for fun on some field (but not to pollute the area) and no one or only very few died it would not count. But if it was blown up with the purpose to kill people and failed it would count as offensive.

Tests don't count, even if they kill many people.

Basically, if it kills many people or does some other harm or was ment to do so but failed, it counts.