In many translations, the word "preacher" is used but others use "herald." In either case, the author of 2 Peter could easily have been copying Josephus' Antiquities of the Jews which does say Noah "preached" to people. However since 2 Peter was most likely written between 100 and 150 CE and Antiquities was around 94-95 CE, it is possible the author of 2 Peter used Josephus as reference. Josephus probably got his information from book 1 of the Sibylline Oracles which was a collection of orations and prophecies muttered by seeresses in a trance but we have little record of who those people were and their predictions were almost always false, but it most likely came from Jewish Midrashim (commentaries) of the Talmud where people assumed the timespan between god deciding everyone was wicked and the flood itself occurred was to warn people. Obviously, the Bible does not support this.
In any event, no official record exists of Noah preaching to anyone, only speculation.
As I and others have stated, even if he did that in no way absolves god from committing mass genocide.
Exactly. You will support any atrocity, even to the level of mass genocide, if god says so.
Tell me, if god came to you right now and commanded you to murder your son, would you do it, Abraham?
How about this, Elisha? Would you side with god if he killed a neighbor boy because he bullied your daughter?
How would you explain to the authorities, Saul, if you invaded a local town god was displeased with and killed every man, woman, and child under god's command?
Could you, Jeremiah, obey and support god if he told you to embargo a city until the occupants turned to cannibalism?
Like I said, you're a terrible person. Worse still, as most wicked people are, you believe yourself to be completely righteous and guided by a higher moral authority.