She took prescription drugs during pregnancy. The amount has never been confirmed. Millions of women do and don't have children who become serial killers. She did leave him, indeed, probably adding to his fear of abandonment. The parents did argue a LOT which Dahmer said he didnt like but he also said he didn't experience any trauma or abuse. He may well have been in denial. Unwilling to speak poorly of his parents. He may just never have realised what it really was but he's dead and we can only go off what he said. If he didn't want to label it as 'abuse' or 'trauma' then that's his choice. The experience of trauma is so individual, what may traumatise one person may not traumatise another. So who knows.
What we've also got to remember is Dahmer will have been comparing the abuse and trauma he inflicted onto his victims against his own childhood. In comparison, your mum taking pills in pregnancy, your parents arguing or leaving you may not seem as abusive as killing and eating folk.
I honestly believe he loved his dad right until the end and didn't wanna say anything bad about him.
Also, just because a parent has a 'mental illness' this doesn't mean a child will be abused. This is such a problematic belief and stigmatising.