I can vouch for the fact that, when you go to the woods for a ritual in natural surroundings, some nosey bugger is guaranteed to come wandering by. Our most awkward time was when we had gone to an ancient outcrop of rocks, high on a hill, at twilight. We had set up the candles and cast the circle before we realised that the Territorial Army was in the same area, on manoeuvres. They kept thundering past us, and we kept singing Happy Birthday so that they didn't wonder about the candles. In the end, we decided to just sit down and enjoy the Cakes and Ale. A pagan ritual inevitably involves Cakes and Ale. We like our food.
I wouldn't be surprised if the people in the photo are making a goth video, though it's possible that they are pagans. Some people like dressing up. I am not familiar with people offering their blood but it's not impossible. Some Christian festivals featured people whipping themselves till they bled, in atonement for their sins, and a few still exist. Some Hindu sadhus mortify their flesh to raise their spiritual consciousness. Pagans tend to celebrate the physical body rather than despise it, but it's possible.
As for the threat... Is that on the video? Otherwise I would wonder, knowing how 14 year old boys are, if the lads had embellished their story a bit to make it more exciting. If it's on the video, I would guess that it's a stupid outburst by an exasperated actor or an equally exasperated pagan. The latter seems pretty unlikely, though. Pagans are fairly well accustomed to intrusions.
As for Satanists, they don't really belong in the pagan camp. They are His Majesty's Loyal Opposition for Christianity.