Memorial Cross For A Mass Killer

When you look back to when Columbine happened the two killers Eric Harris and Dylan Klebold had memorial crosses made by a guy from Aurora IL I believe. I personally think its a stupid thing to put a memorial cross for a deceased perpetrator of a mass murder next to memorial crosses of their murdered victims because it will cause outrage and it makes sense logically that these killers weren't victims of their crime despite committing suicide or suicide by cop and shouldn't be with their victims. My main question is why did killers get crosses in the first place? Why did they stop doing it and when did they stop doing it to be specific? I remember the NIU shooter having a memorial cross but a bunch of college hooligans burned it down from a book about the shooting. Feel free to write down any other examples of these monsters getting memorial crosses.