Manson is definitely crazy, probably still dangerous, and certainly guilty. Probably of murder.
I'm not all that familiar with the details, honestly, but I know he didn't actually kill anyone. The question then becomes, did he brainwash his "family" and use psychological force to get them to kill for him? My understanding is that he did.
If I brainwash someone and they kill for me, then the argument can be made that they had no choice in the matter and that, at the time of the murder, they were just a tool. They were the "gun" that I used to kill another person. So the extent to which Manson took away their free will is the same extent to which he is guilty of murder.
However, it isn't quite as easy as that. I imagine there is some uncertainty as to how much control these people had over themselves both before and during the crimes.