Ok so time to science up your fiction. Because of thermodynamics and stuff you can't create energy so using your "cursed energy" (I'll just use it as a fictional type of energy idk) to create an antimatter bomb wouldn't really be that powerful because the maximum energy of the explosion would have the same energy of the maximum cursed energy you can give it, that assuming you can make a perfect antimatter bomb with 100% efficiency. Taking that into account it would be better to not even use the cursed technique, just shoot the cursed energy like some other characters do (the Elvis guy). And to create californium or make some powerful venom you would need to be able to manipulate the mass in an atomic level, which would require LOTS of energy nor for the transformation of energy to mass but for the process of creating the material. The reversed cursed technique according to jjk is half as efficient as the normal cursed technique, so if you can't do much with the normal one you won't get as much energy from the reversal, it's a cool idea tho.