The mark for death is the number 6 and is the number of a man. The number 666 is just 6 three times to affirm completeness. Having the mark of the beast, the mark for destruction, is just those people looking without reservation to the teachings or doctrines of men to save them.
[Mat 15:7-11; Col 2:20-23] They have joined a cult and are drinking the Kool-Aid of religious doctrine. People who are able to buy and sell and take full advantage of life in this old world system are those who have chosen to make doing those things (use of their right hand of power to conduct business and gain wealth) the most important thing in their life. It is thus the mark in their forehead. Those who have made the teachings of Christ the most important thing in their mind are marked in their forehead to gain life.
[Eze 8:17-9:7]