Mankind's archetypal crime, Cain's murder of Abel, points to a persistent flaw in human nature: It is always easier to tear the other guy down, than to try to improve yourself.
The consequence in politics is that it is easier to erase the progress of others, so as to give yourself a lower bar by which to be judged. It's hard to stand tall among temples and skyscrapers; easier, on a plain filled with rubble.
G-d showed mercy to Cain, and what's the thanks He got? Mankind devolved into violence. The mark of Cain, meant to protect the killer from arbitrary mob justice, became a status symbol. Four generations later, Lemech could brag, "I've killed a man for wounding me, and a boy for bruising me - if Cain was avenged 7 times, then Lemech for 77." Mankind had perverted G-d's compassion into a literal license to kill. No wonder He's pissed.
After the Flood, G-d decrees that there are going to be some new rules. "Who sheds the blood of man, by man shal his blood be shed."