How about some band brought a chemical on stage to shoot it into the crowd just for show but that stuff turns the people over night into zombies and the band has to make up for it and join the fight against the hordes...

Since the festival takes place in the countryside there should definately be farm machinery involved when fighting the zombies. They could ,like, A-Team those things up, attach chainsaws and whatnot and plow through the masses, literally!

The showdown has to be infront of the mainstage! A band is playing to lure all the zombies into one place for the good guys to just deal with them in one giant massacre!

Some guys on the campground do some satanic ritual just for fun but end up summoning a real demon that starts to turn all metalheads into mindless zomie servants to take over the world!