After everything billy has done over the years, not even treating max nicely could redeem himself. After over 6 years of verbal mental and most likely physical abuse, just a sorry or treating her like a decent human being wouldnt be able to solve everything hes put her through. Look, i get now that you see his past and think “oh he was abused i feel bad for him” he still became an abuser and its best not to feel sympathy since he still projected his emotions onto a 13 year old girl. In the end, as the abused who became the abuser, his death was the only way he could redeem himself.