Complex sentence: Darren then scares himself away so he wouldn't become a vampire in the future and gives his diaries to Mr. Tall to give to himself when he's older so everyone will know the truth of what happened.
Split into simpler sentences: Darren then scares himself away so he wouldn't become a vampire in the future. He also gives his diaries to Mr. Tall to be given to Darren when he's older so everyone will know the truth of what happened.