Witch oil is refined from soul energy and is one of the most powerful fuels known.  Philosophers argue about the ethics of burning it because some people believe that doing so destroys the souls contained in it, and refining it traps innocent spirits from reaching the afterlife.  Others don't believe in souls and think that those who argue otherwise are superstitious at the expense of progress.  Witch oil may or may not be evil, depending on one's beliefs about it.  The oil may or may not absorb souls, but at the very least, it has been scientifically verified that the substance gains its potency through specks of light that begin floating in it immediately after a sentient creature dies near it. Witch oil is highly volatile and an observer can hear the sound of shrieking people when it's burned.