Commentary for Genesis 50:19

כי התחת אלהים אני FOR AM I INSTEAD OF GOD — Am I perhaps (do you think that I am) in His stead? The ה of התחת expresses a question. Even if I wished to do you harm would I at all be able to do so? For did you not all design evil against me, and you did not succeed because the Holy One, blessed be He, designed it for good. How, then, can I alone, without God’s consent, do evil to you.

התחת אלוקים אני, while it is true that I am a judge, a surrogate of His, charged with the task of carrying out His decrees, and I could punish anyone who was empowered by Him to carry out His decrees. This is analogous to the principle that one court must not annul the rulings of a previous court on the same subject.” (Eduyot 1:5) It is true that you had been charged by G’d to carry out His decrees, but you did not send me here deliberately but G’d. (Genesis 48:8)

כי התחת אלוקים אני, "Am I then in G'd's place?" Joseph meant: "if you are guilty it is up to G'd to punish you; it is not up to me to revenge myself or to hold a grudge against you." Joseph may not have told the brothers that he had forgiven them because under Noachide law the matter did not depend on his forgiveness at all. Under Noachide law, once a person has committed the sin of kidnapping or robbery, he is guilty of the death penalty. The fact that the victim forgave the criminal is irrelevant to the penalty. Joseph added that he had to interpret the brothers' actions in a manner which reflected credit upon them.