Defining evil loosely as "opposed to God's will," then: God's stated will was for the human race to "Be fruitful and increase in number; fill the earth..." Genesis 1:28
He had a specific number of humans in mind to whom to offer salvation, and they were preventing their birth by their violence pre-Flood, so God had to start over. Once He did, the Tower, representing humans' self-will, continued that defiance: by remaining at Babel and refusing to "fill the earth" as ordered, again they were preventing, by their geographic proximity, the birth of the billions He planned.
The earth was inherited in thirds by Ham, Shem and Japath descendants. Nobody was preventing births because they were the only people on earth. Related people would naturally group together. There were cromagnons and Neanderthals and lions and tigers and bears and crocodiles wandering around waiting to eat stray loners. And the collective memory of The Flood was probably on their minds, too.
Once there were enough people, The Almighty disbursed them to “fill the earth,” by gifting them courage to face the unknown. By assigning them language groups, who would head off in different directions like the Oklahoma Land Rush, the groups had a higher survivability chance than a loner would. Each group would be large enough to build more than one city. And then hundreds of cities. The more people trusted Him, the more courageous they were in pushing into new lands.
A rainbow was one thing, but the miracle of the tower - the gift of courage - was a huge blessing. There couldn’t have been a more concrete reassurance from The Almighty that there would be no more Heavenly exterminations. People truely could, go forth and multiply. Trust and courage is what spreads Faith. Just as The Almighty planned all along.