Well, as someone who's extremely interested in Jeffrey Dahmer, I'm gonna try to break down the possibilities. To begin with the period of time which both crimes took place plus the nature of it is a huge huge difference.
I think at that point of time people were extremely afraid with what Jeffrey has done, it wasn't a school shooting, he was basically a cannibal and he targeted a certain minority of victims. With addition to still being alive, yes, he was there to take ALL of the blame, the name callings, how much he was a monster etc and he took it with open arms. He knew what was coming, and he didn't care. He wasn't a seventeen year old high school student with the most average suburban life with friends. Jeffrey was grown, he was a loner, a gay man, and he killed and ate his victims.
I'm not refuting the fact the Lionel most definitely must have gotten alot of flack and comments back then, I wasn't there when it was a hot media sensation to follow along the progression of the case, I mean he did write a book, I'm sure people were giving him hell about not being a present parent, leaving jeffrey alone at their house during the separation and possibly even sexually abusing Jeffrey, but it got swept under the rug pretty swiftly. I think I leave it to the fact that Jeffrey was alive and present to have the focus and projections go to him. He was still there and they analyzed him to the bits and Jeffrey was eloquent and very ready to explain why he did what he did.
"because people accepted that it was a mental condition that wasn't his fault?" I think so too. I'm not sure what they diagnosed him with, but I strongly remember during his speech in court he said something like, feel free to examine me and diagnose me with whatever mental crisis I have, i admit the bad things that I have done and let this be an example so as to ensure a crime like this never happens again or something like that...
So yeah, the perpetrator was there to take all the rap. Dylan wasn't and, Sue could've hidden like the Harrises did but, she would rather come out and openly acknowledge it as a mother of a school shooter and share her story.
Oh no, I don't think Jeff left any signs at all. People thought he was a simply a quiet brooding loner. and It didn't matter if he left signs because people neglected the shit out of him and at his time nobody cared and weren't aware. Infact Jeffrey did check himself into a mental institution at one point of time because HE knew he had a problem...it didn't work cause he got out of there real quick and just went back to his habits.
Jeffrey didn't even want to be a murderer, he had to get super drunk to kill, he was a repressed homosexual who didn't want someone he fancied to leave him, and to have them morbidly stay it meant he had to kill them, drill acid into their brains, keep their corpses in his apartment, and have sex with them dead or with their body parts/consuming them.
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Will you remain my fire in this temporary paradise?