Midrash su Numeri 35:12

"he shall be put to death": in beth-din. You say this, but perhaps it need not be in beth-din. It is, therefore, written (Numbers 35:12) "and the slayer shall not die until he stand before the congregation for judgment." What, then, is the intent of "he shall be put to death"? In beth-din.

(Bamidbar 35:12) "And the cities shall be for you as a refuge from the avenger. And the slayer shall not die until he stand before the congregation for judgment.": What is the intent of this? Because it is written (Ibid. 27) "And if the avenger kill the slayer (outside the city of refuge), he has no blood" (on his hands), I might think (he may kill him even) immediately. It is, therefore, written "And the slayer shall not die (at the hands of the avenger) until he stand before the congregation for judgment" (and is pronounced an exile.)