Why was the flood an act of mercy to unborn generations?
The flood was an act of mercy to unborn generations because pre-flood society was filled with such violence that children born then would soon be murdered. By cleansing the slate and starting over, God spared future generations from that relentless killing, so the flood was not wrath but mercy.
Les explains that even though the flood was awful, it was not the wrath of God. God was brokenhearted because everything He had begun had fallen apart and turned in rejection. The flood was an act of mercy to unborn generations because people were killing each other so fast that a child born before the flood had no idea how long he would live before being murdered; the world was filled with violence. So it was not so much wrath as a cleansing of the slate and starting over, and the flood was almost instantaneous destruction.