When there is 15,000 – 20,000 murders PER YEAR (compared to a Jewish Beis Din having ONE death case per SEVENTY [or seven] YEARS)
I think you read that wrong. No where does the Talmud state that a Bais Din had only one capital case in seven(ty) years. The correct reading is that a Bais Din that *carried out* a capital sentence in that time period was called a “murderous Bais Din.”
Considering the near-impossibility of actually imposing the death penalty under Bais Din (how many criminals do you know that are going to respond [as required] “Yes, even so I’m going to do it” to a warning?) and given the statements by other Tana’aim (was it R. Akiva who stated that if it were up to him no one would be executed?) it is, indeed, a wonder if a Bais Din actually executed one person in seven years.
But that does not mean that they only had one case in that time. I’d venture that they had many cases and that the vast majority resulted in an acquittal (or, perhaps, lacking a proper warning, witnesses and the like, were never brought to Bais Din in the first place.)
The Wolf