When omniscient God had a plan for Jesus, why did God think of killing everyone?

There seems to be a contradiction between these two verses. > So the LORD said, "I will wipe from the face of the earth the human race I have created--and with them the animals, the birds and the creatures that move along the ground--for I regret that I have made them." Genesis 6:7 > “He paid for you with the precious lifeblood of Christ, the sinless, spotless Lamb of God. God chose Him for this purpose long before the world began, but now in these final days, He was sent to the earth for all to see. And He did this for you.” 1 Peter 1:19-20 In Genesis, God judges mankind and destroys them, seemingly thinking that will fix them. In 1 Peter we're told that salvation through Jesus was the plan since the very beginning. So the motivation for the flood doesn't make sense in this light. How is this apparent contradiction typically explained?