why I think he's guilty (+ I never read the book)

You never can say 100% that he is guilty, something could come up but...

1. These drug crazed hippies never killed or showed up again? If they were some kind of Manson family types, they would kill or break the law somewhere else & be caught. The Manson family bragged about the killings and were very sloppy about covering up evidence. Too incredible to believe they never killed again & covered their tracks so well. Seems odd they'd go after a military family rather than the more submissive suburban families out there. And if they did ID the girl with the hat, was she ever in trouble with the law or linked to crimes?

2. Why would the hippies cut up the wife & kids but stab the dad 2 or 3 times only? He's the authority figure & the biggest threat there, they cut him to pieces.

3. The girl saying "acid is groovy" is so a square trying to think of what a hippy would say.