Complex sentence: He then walks back to his house but decides to sneak into the basement irrationally fearing that his father will kill him because he is late and fears that if his father hears him, his father will think he is an intruder.
Split into simpler sentences: He then walks back to his house but decides to sneak into the basement, fearing irrationally that his father will kill him. He is late and fears that if his father hears him, his father will think he is an intruder.