Complex sentence: Breen went to Stoval's house, and when she threatened to expose Breen's affair, he killed her, by suffocating her with a pillow, and burned down her house, making it look like she died falling asleep while smoking a cigarette.
Split into simpler sentences: Breen went to Stoval's house, and when she threatened to expose Breen's affair, he killed her. After suffocating her with a pillow, he burned down her house, staging the crime scene in such a way as to make it look like she died falling asleep in bed while smoking a cigarette.