The goth "movement" - which is a misnomer - is a style of music (often moody, melodic, slow electronic music with an abundance of minor chords - not Marilyn Manson, but bands like Bauhaus), a style of dress (wearing a lot of black, copying Victorian styles and 19th-century arcana) and a social group. It is not an exclusively American movement.
Satanism and the goth movement are not synonymous. Many "goths" are teenagers, to whom satanism has a certain rebellious appeal, but are not in any way dangerous people.
If you're going to link these boys with a subculture, the militia movement is far more accurate, with its obsession with war games and guns, than the goth movement. Most "goths" I have known spend their time writing abysmal poetry and traipsing through graveyards, trying to look poetic and pale.
They are mostly social misfits who can find acceptance in a subculture with very different social norms from the rest of American teenage society. What better way to rebel against the sunny disposition of the jock/cheerleader stereotype than by cultivating a calculated depression?
Seattle, Washington, USA