Complex sentence: It was the deadliest accident involving a Boeing 737-300 at the time;, it falls behind China Northwest Airlines Flight 2303, and is also the accident having the highest number of fatalities involving a China Southern Airlines aircraft.
Split into simpler sentences: It was the deadliest accident involving a Boeing 737-300 at the time;, it falls behind China Northwest Airlines Flight 2303. It is also the accident having the highest number of fatalities involving a China Southern Airlines aircraft.