I have been thinking, and I have noticed something. The man in black has caused a good amount of character deaths.
Boone Carlyle- Locke saw the "bright light" or man in black, and started believing about his destiny. He later recruited Boone, when they found the hatch, believing that it was destiny. This led to Boone's death.
Shannon Rutherford- I think it is possible that walt was actually a manifestation of the man in black.
Eko Tunde- Obvious
Charlotte Lewis, Daniel Faraday, Juliet Burke- The man in black posed as Christian and told Locke to move the island. This caused Ben to turn the wheel, thus starting the chain of events that would cause the death of Charlotte, joining Dharma, Faraday returning, death of faraday, jughead plan, and Juliet's death.
Sun Kwon, Jin Kwon, Sayid Jarrah, Jack Shephard- Obvious