True story: I did something similar with my parents, and we had a heckagood time. We did it in December. We did it through snowy mountain states. It was actually really fun, and it remains a happy memory for me.
I can't speculate on reasons BCK may have had for the trip because IDK and respect to WS, nor do I know his relationship with his father, but IMO JMO being a homicidal maniac isn't necessarily the only reason someone would do this. The reason I would give my parents, "come on, it'll be fun!". And because we all shared that genetic disposition, they'd have said, "you're right! Let's do it!"
It's only in retrospect that BCK's trip raises red flags for me. AND IMO JMO, the opportunity to hang with your parents and have an adventure should not be done only after a quadruple homicide. I'd say take the opportunity when you have it, and don't murder anyone first. JMO IMO IME
editing to add: IMO JMO this may be a likely defense explanation, too IMO