I've given this motive thing quite a bit of thought, and here's where I'm at:
1) Helter Skelter
2) Copycat... i.e., spring a "brother" out of jail
3) Manson's mental state and bitterness towards society
4) Drugs
5) Leno's gambling debts (at the LaBianca site)
I really believe, all of the "family members" were "mentally compromised" by the time of the murders, and each had a slightly different mindset.
I'm sure there were several "stories" circulating at the ranch. Some of the kids believed the "HS" story (motive #1). Some of the kids believed the real goal, was to spring Bobby out of jail (motive #2). And Manson himself, was mentally disturbed and bitter towards society (contributing factor or motive #3).
That covers motives #1, 2,and 3.
I believe those three motives, may have been coupled with a drug motive (i.e., motive #4)... which only Manson and some of the men were privy to.
(This would be the elusive "real motive", which everyone has been seeking for 4 decades).
There's also an outside possibility, that Leno had his ass in a sling over gambling debts (motive #5). And again, this is something that only Manson and the guys, would have been privy to.
So, we have a minimum of 3 "motives" (contributing factors)... and possibly 5.
Look:
The girls believed whatever they were told... and I really believe, they were told conflicting stories. (i.e., "Helter Skelter" and "Free Bobby").
As for Charlie, he was crackers. That was a contributing factor.
No matter how you slice the pie, he was (as John Lennon said) "cracked".
I'm starting to agree with Leary. Beyond sheer curiosity, seeking a motive at this point is probably an exercise in futility.
In the words of Harold True: "You can't ascribe reason to crazy people."