Complex sentence: She then uses her mind to break 220-volt electricity cables backstage and throws them crackling on the wet floor to electrocute the prom-goers and burn down the gym, leaving everyone inside (including the unconscious Tommy) to be burned to death.
Split into simpler sentences: She then uses her mind to break 220-volt electricity cables backstage and throws them crackling on the wet floor to electrocute the prom-goers. Carrie leaves everyone inside (including the unconscious Tommy) to be burned to death.