Alcohol + benzos can potentially kill you if you go for a swim while drunk and high, you may drown without realizing it. Other than that, no.
Barbiturates like Nembutal have been replaced by benzos specifically because overdose side effects included death. Benzos won't kill you, but withdrawal from benzos (stopping taking them abruptly) may. However, benzo withdrawal is horrific if you've been a regular user of them in high doses, you do not want to go that way.