The Devil, who tries to oppose God and does these sort of things for fun.
Yes Free Will plays a part here, but some argue that all are suject to God's Will no matter what. In that case, God has a weird sense of humor and works in mysterious ways. I cannot explain then, why lightning bolts got thrown at those people. They must have moved into an electrified charge area.
Maybe God was at work trying to save those people from the first few bolts? Why he didn't from the final bolt is beyond my reasoning.