A man's mind can influence his body profoundly. If allowed to function viciously and entertain unwholesome thoughts, it can cause disasters, even kill a being. In the case of Breivik, he used meditation to reinforce his hatred so much so that he was able to kill many people viciously.
Dear Ensapa, this type of meditation is not Dharma at all. Meditation used in this way is definitely a "tool" but it is not a "sacred practice" and Dharma is definitely not a "double edged sword". Dharma is the Buddha's teachings and therefore cannot be unwholesome. It is your choice of words that is misleading.
When the mind is concentrated on unwholesome thoughts such as hatred, jealousy etc., it is very harmful to oneself and to others. Dharma is exactly the opposite. Dharma is controlling the mind and keeping it on the right road so that it becomes useful for oneself and for society. Dharma is concentrating the mind on right thoughts.