The Force is Strong With This One
You might overestimate the role of (loving) nurture. The fact is the most important factor is not nurture, especially parents nurturing, but nature. Countless loving parents have had abusive kids, countless abusive parents have had kids that didn't become oppressors.
Learning involves efforts, the fear of the unknown, the fear of failure, the fear of change. Learning opposes entropy that is a big part of human nature. So, in order to learn, one must suffer. Suffering is necessary but it is not always sufficient. Too otfen, the one who suffers doesn't even learn.
Can you really imagine a world only made of love, beauty and peace? Could you really appreciate love if you didn't know what hate is? Could you truly appreciate beauty if you didn't have encountered ugliness? Could you fully enjoy peace if you had never faced war?
You're right when you say you can not know what is sweet until you try bitter. But what I am looking at, not in the world, but concrete in the everyday example before my eyes is that people who have experienced terrible brutality (talking specifically about the consequences of the war in my country) and who survived this torture, today have in themselves terribly full of hate (hate against an enemy on a national basis) and this hateis further passed on to their children. Rarely are those who have done terrible crimes to completely forgive. Look at those troubled children ... what will they spend on next generations?
This is just one of the thoughts, I'm not saying you're not right either