Manson was basically a 'spiritual guru' back in the day, and ran a semi-cult style organisation that appealed to many musical stars at the time. He was friends with the Beach Boys, Steve McQueen, and they would frequently hang out together.
With many of the current bands, their names are 'subtle' referrences to more sinister issues. Kasabian...as the original post says, is Charles Manson directed. Kaiser Cheifs...Kaiser Wilhelm II, whose death started WW2. Marilyn Manson is named after the 'entertaining dichotomy' of the names of 2 of the biggest names in the 60's, Charles Manson and Maril