if it can be used to murder people it will be.
the only reason we dont have antimatter weapons or gravity guns is because we haven't figured out how.
We have international treaties that ban biological and chemical weapons. Other weapons that are regulated include anti-personnel mines, cluster bombs, and blinding lasers. Expanding bullets are banned in military uses as are incendiary weapons against civilian targets.
That's the state today. Throughout history there's been a long negotiation about what weapons have been allowed in combat.