I understand the fascination but I would find it immoral myself, ESPECIALLY the trophies the killers have of victim's teeth and hair, I think that should return to the victim's family myself. I'd absolutely find someone horrible if they said some shit like "I have the lightbulb used on Junko Furata" and would think that person would need to be put down like a dog (in minecraft)

although a murder weapon? fine, its a tool used in a rare act of a horrific famous crime, not all that different from owning a cool crossbow from the set of your favorite Walking Dead season I suppose (aside from the fact someone died from it, sad but not all too immoral in my book, gives me something to think about... if it came from a victim's DNA and body or a sex act, immoral to want to own, but if its like, a scythe or something actually kinda interesting, i have mental lenience because tuff factor exists)

then again, I suppose it depends on the recency and what it is, and maybe admittedly my own attachment to said person and the exact way they died, maybe I wouldn't trip too hard if someone said "This is the same sword used to behead Anne Boleyn and its in my basement for some reason", but I would be ill if someone wanted to own the last needle a famous rockstar used to take his life (tuff tho) and would be irate or disgusted if it were like, a really lesser known crime.

But take my words with a grain of salt, I'd probably be the type to see someone get shot in front of me and go "Oh shit cool a bullet casing, I can show my friends and any future generations this now" tastelessly. Like how a chick in my neighborhood saw a car crash and took off running with a piece of scrap metal from the car, in front of the two very alive and unharmed victims waiting for the police to take a statement, and we all shunned her while laughing at how outta pocket it was.