How Your Evil Thoughts Act as a Magnet to Attract Like Thoughts from Others

A good thought is thrice blessed. First, it benefits the thinker by improving his mental body  (Manomaya Kosha). Secondly, it benefits the person about whom it is entertained. Lastly, it  benefits all mankind by improving the general mental atmosphere.An evil thought, on the contrary, is thrice cursed. First, it harms the thinker by doing injury to his mental body. Secondly, it harms the person who is its object. Lastly, it harms all mankind by vitiating the whole mental atmosphere.

Every evil thought is as a sword drawn on the person to whom it is directed. If you entertain thoughts of hatred, you are really a murderer of that man against whom you foster thoughts of hatred. You are your own suicide, because these thoughts rebound upon you only.

A mind tenanted by evil thoughts acts as a magnet to attract like thoughts from others and thus intensifies the original evil.

Evil thoughts thrown into the mental atmosphere poison receptive minds. To dwell on an evil thought gradually deprives it of its repulsiveness and impels the thinker to perform an action which embodies it. 

Source: Pgs. 138-139, Mind --- Its Mysteries and Control by Swami Sivananda