The spear was metallic and deep inside the bull, so lightning entered the bull's body through it and began to fry its internal organs. They caused another round of pain for the bull, but it didn't die.
The body of a rank 3 beast is very strong even if it isn't enhanced with life force. This bull, for instance, has a body that is around the 160th transformation. That means its body is at least 16 times stronger than the body of a rank 2 beast.
Not to mention that the injury that he had caused so far is not enough to kill a rank 2 beast. It is definitely not enough to kill a rank 3 beast.
The best that he has managed to do is burn its internal organs. But even if he removes all of the bull's intestines, as long as he doesn't damage its heart, the beast won't be fatally injured.
Arthur knew this, but he thought that it was better to stay away from the bull's heart for now because it is too close to the bull's head, its horns, and its snapping mouth.
