Six may be a "perfect number" mathematically, but there is an equal body of tradition indicating it as a number of evil. To Christian numerologists, seven is a number of God and perfection. As six falls one short of seven, it stands for imperfection and thus sin or Satan. Get too many of these together and you get 666.

At the same time, six counts the points of the hexagram (whether you take it straight or with a twist) and the sephiroth of Microprosopus, so it can't be all that terrible, ne?