How can people change like that? (SPOILER)

The grandpa was depicted as a meek, caring, good old man, and the whole town respected him. But it turns out he was an asshole, a drunkard, a violent guy when he was young.

How can a man change so drastically like that? I had an uncle who is like that. When I was a kid I wouldn't believe if anyone said he was a wife-beater. He gambled and basically was never sober in his whole younger years. He put his wife and family in miserable condition. They were poor and had to work really hard because of him.

Yet when he got old, he became a really nice old man. Family seemed okay and the kids all grew up fine. Two are architects and one is a quite succesful bussineswoman. She's the richest among the siblings. Which according to the family stories I heard, she's the one treated worst being the middle child.

Human beings are weird.