Matthew Poole and continuators on Genesis 39:20

20 And Josephs master took him, and put him into the prison , a place where the Kings prisoners were bound: and he was there in the prison.

Quest. Why did he not kill him, the crime being capital and he having so undoubted a power in his hand to do it? Answ. 1. It is probable, he was a little moderated by Iosephs Apology, which doubtless he made for himself, though it be not here recorded. 2. This is to be ascribed to the good Providence of God, which restrains the waves of the Sea, and the passions of men, and sets them their bounds which they shall not pass, which watched over Ioseph in a peculiar manner.

Traitours or great offenders against the King, whose Prison doubtless was none of the easiest, and therefore it is called a Dungeon, Gen. 40. 15. and 41. 14. and he indured great hardship in it. See Psal. 105. 18.