500mg diazepam plus alcohol could ABSOLUTELY kill you. Benzo's alone are almost impossible to OD on (a nurse once told me - um, after a benzo OD, so...she really shouldn't have been telling me this? Like, looking back, it was almost her telling me how to successfully KMS? - that you need to mix benzo's with another CNS depressant for them to dangerous (ie GHB, alcohol, barbiturates or opioids) as in order to die from a benzo OD you'd have to take so many of them so quickly that you'd choke to death on all the pills before you OD'd.
But mixing diazepam with alcohol is very dangerous. I've blacked out from taking just 20mg diazepam along with my usual alcohol intake (which was admittedly about 6 bottles of wine a day at the time).
They don't just add together, so that for example, 6 beers plus 20mg diazepam = 6 beers plus 20mg diazepam, they can as much as double each others effects so 6 beers plus 20mg diazepam can = as much as 12 beers and 40mg diazepam.