The Rosh seems to be bothered by the fact that the Torah uses a similar language when G-D created Adam in Chapter 1, verse 26: “And G-D said, ‘we shall make man in our image like us…’” The Rosh there explains “in our image” to mean in an image which is unique to him by us. This is referring to the image on “Hashem’s Holy Throne,” a face of a man which is shown to a person at the time of prophecy. “Like us” is referring to the fact that man is slightly similar to the celestial beings, i.e. angels, to be able to differentiate between good and bad…” (Click here for Hebrew text.)
The Rosh then explains why Kayin and his descendants did not reach the levels of “made in his image and likeness;” even though they too stemmed from Adam, it is because “they said, ‘we shall remove ourselves from G-D.’” It seems from this statement that they knew the undeniable truth. There were walking, breathing, upstanding individuals who glowed with purity and righteousness, but the majority of people in the world, for whatever reason, chose to not walk in the ways of Hashem and do His will. Yet they could not just deny the truth and what was moral; they had to verbally tell themselves that they wanted to remove themselves from G-D. That was the only way they could become so evil. It was not out of ignorance, or laziness, or just an unwillingness to acknowledge the difference between right and wrong. They had to actively remove themselves from what was clearly right by telling themselves to move away. Only then were they able to digress into such a rut that Hashem reneged on His creation and essentially started all over again with Noah and his family.
It is possible for a person to know and understand the undeniable truth but still do the wrong thing. But in order to do so, the individual would need to actively remove him or herself from the correct path, for whatever personal reason, in order to totally stray.