Usage patterns
凶器を使うto use a weapon (in committing a crime)
The perpetrator used a knife as a weapon.
凶器で〈人〉を襲うto attack someone with a weapon
The man attacked the store clerk with a weapon.
凶器が見つかるfor a weapon to be found (e.g. at a crime scene)
A weapon was found at the scene.
凶器として使われるto be used as a weapon (often of an ordinary object)
Even an umbrella can be used as a weapon.