Wolf-
In a case where a murderer does not meet all the halachic requirements for receiving the death penalty (e.g. only one witness, or a minor flaw in the hasro’oh) beis din would toss him in the brig and throw away the key. Not acquit.
Certainly not universally.
Of course, that also then refutes BT’s point about BD implementing a death sentence once in seven(ty) years. Ultimately, does it matter if they killed him through halachic or extra-halachic means?
The Wolf