Manson preyed on the weak-minded, searching-for-something, Daddy-didn't-love-me, lost souls - the ones who were inclined to believe anything (the weirder and more out there, the better) who could be easily led... the runaways, the ones who just wanted to be part of something and feel they belonged. He'd spent more than half his life in reform school, jail, or prison, and knew how to spot the weak ones in the herd and single them out for his con-artist/mind games. He wasn't much different from Jim Jones or David Koresh. He knew how to spin a good tale to that type and get them to believe he was The Chosen One they wanted to follow. As others have mentioned, drugs and the culture of the time made it all even easier.
I don't believe Charlie was insane. He certainly had a lot of issues and was damaged goods from the abusive childhood he'd had and the lack of anything approaching a normal early life, but wasn't actually insane. He knew full well what he was doing and was basically lashing out at society for the life he'd had and for not recognizing his "greatness" and talent. He was basically just a cowardly, narcissistic con artist without a conscience.