Like you, I have only read Boyhood Island and like you, I am at a loss to understand his mother's attitude.
I had a very similar father to Karl Ove's, not sadistic but violent and with an uncontrollable temper. What really resonated for me was how Karl Ove never knew what he had done wrong; what had precipitated the violence. So familiar: stuff I had buried.
I left home as soon as I could legally, aged sixteen and my younger brother did the same. My mum spent the rest of her life trying to reel us both back in: a high price for her non-intervention.
But in fact it is Karl Ove's final observation, on almost the very last page about the boys and the sexually aware, beautiful young girls; what happened to them all? Did they become actresses or models? Porn stars maybe? No, they were blue-collar through and through and never got away; married teachers and bus drivers and had two kids and are still there, raising their own children. I hope and pray that they are doing it with more kindness than Knausgaard's dad.