Question: Were killing, adultery, stealing, etc., not sinful prior to the Ten Commandments? God destroyed the earth with a flood because mankind was so wicked, or evil. Evil based on what?

Question: I heard a pastor/teacher on the radio state that when Joseph refused Potiphar's wife's sexual advance, he was a godly man and honoring God. That makes sense, but he also said that if Joseph had given in to her it would not have been a sin, since this was prior to the Ten Commandments. Were killing, adultery, stealing, etc., not sinful prior to the Ten Commandments? God destroyed the earth with a flood because mankind was so wicked, or evil. Evil based on what?

Response: One might very well wonder why Joseph asked, "How then can I do this great wickedness, and sin against God?" (Gn 39:9). We are also reminded of Romans:5:14Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come.
See All...: "Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come."

"Death reigned from Adam to Moses." In other words, there was a standard by which one could judge what was sin and what was not.

It is instructive that even the most isolated tribal groups have distinct concepts of what is sin. In Romans:2:14-15 [14] For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: [15] Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another;)
See All... we read, "For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law to themselves: Which show the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another." According to Scripture, there is a "law written in their hearts." This explains why there are universal prohibitions throughout the world. This is certainly not an assurance that men will fulfill that law, but in the mercy of God he has caused His law and standards to be revealed to humanity.

Consequently, Joseph (before the law) stated, "How then can I do this great wickedness, and sin against God?" He knew very well that what Potiphar's wife proposed was sin, and if he yielded to the temptation, he would be sinning.