To make into a weapon.
Anything can be weaponized. A big enough rock, dropped from a sufficient height, is a very good weapon.
To make more effective as a weapon.
To weaponize anthrax it is made more distributable, not more virulent.
Made into, or to serve as, a weapon.
Weaponized anthrax is a biological terror weapon, whereas anthrax found in the wild is unlikely to cause a plague.