1. Black Magick's aim is to harm yourself or another either purposely or accidentally. Some magicians feel that it would not be working to hurt someone else, except that they, themselves will suffer as a consequence. "As you sow, so shall ye reap" is not merely a philosophical aphorism to a magician, it is an actual physical fact. Most magicians do not want to chance getting "zapped" by the law of Karma for a foolish act of Black Magick.
SOURCE: The Truth About Psychic Powers, Donald Michael Kraig
2. Magic performed to cause harm. Louis Culling’s point about "it is prevailing mental aberration of the ignorant to label as 'Black Magic' the pentagram with two points upward" is that it not the tools of magic that are evil but only the intentions of the magician. A pentagram is a pentagram no matter which end is up; a knife is a knife no matter how it is used, and a gun is a gun no matter it held by a policeman or a murderer.