[2:73] so We said, 'Smite him with part of it'; God brings to life the dead, and He shows you His signs so that you might understand.
Allah (s) wanted to give life to the dead man so that he could expose his killer through the testimony [of the slain], so He commanded them to kill one of their animals, making it the cause of bringing the slain man to life. The allusion in it is that anyone who desires the life of his heart will only attain it by sacrificing his lower self, since the heart of anyone who sacrifices his lower self by difficult efforts (mujāhadāt) comes to life by the lights of witnessing (mushāhadāt). Likewise, when God wants someone's reputation to live forever, He causes his reputation in this world to pass into obscurity.