It's possible that "what was the motive of the vandal/were they WT or ex?" may be overthinking it a bit. In my experience, graveyards are subject to vandalism as a normal course of events. They tend to be open and accessible and usually unguarded, especially at night. The pyramid, by virtue of its prominence, could be a target for someone out to make their mark by being destructive. I'm not saying it couldn't be the other two alternatives either, just that there is also this third possibility.
Not being the vandalising sort, I'm sad that this piece of history is no longer available for viewing. Two decades ago it was on my must see list if I ever got to that part of the USA.
Just to make it clear, it wasn't the only historical oddity on that list. Grizzly Adams' grave was also one I wanted to see and photograph. That's in Massachusetts, but if I was touring the North Eastern US, both could have been doable.
I'm still not ruling out the other two possibilities. Slimboyfat's theory of a disgruntled exJW taking things out on a visible monument to the foolishness that encompassed his life at one point isn't a bad one.
moomanchu, surely that's a waste of good coffee? 😳