Complex sentence: After a few years, Brian finally went too far and killed Rebecca; he was then placed in a mental institution for the next 15 years, while Bruce went to stay with his Aunt Susan, becoming a surrogate family to him from then on.
Split into simpler sentences: After a few years, Brian finally went too far and killed Rebecca. He was then placed in a mental institution for the next 15 years, while Bruce went to stay with his Aunt Susan, becoming a surrogate family to him from then on.