When you have a puupy, the most early training is through fear. You swat the newspaper. You yell "NO''. The first few months you must use drastic noisy maneuvers to get his attention and enforce discipline. Thereafter, once he gets the message and learns a few basic disciplines, you begin to reward with a little affection. As time goes on the affection becomes most important and any discipline that needs to be enforced is done by withholding affection or reward for a short time.
Sort of the same with a small child. You have to get their attention fast when they may be about to make a dangerous move. You yell, you startle with a spank, etc. Children are basicially curious and selfish. It is not until they develop further that they have a conscience and can understand love. You have to instill conscience in them when they are young.
So, suppose evolution is true? Perhaps when man was still in his somewhat neanderthal brain, the only influence he understood was fear. The old testament seems to be all about fear. The God of fear. But perhaps as mans brain and conscience became more developed, he was able to grasp the disciplines of love. New Testament is all about love.
So its not so much that God is angry. He maybe just used what works on neanderthal man. Maybe that, too, was only supposed to be for a time and season. Until man had developed more socially.