IMHO I think a lot of it depends on the type of crime and who the perpetrator is, their motive etc.For example, a spree killer might use the same weapon for all their crimes. IIRC the Zodiac used the same gun for multiple crimes.
If BK hadn't left the sheath behind, he could have easily kept the knife without anyone connecting him to the crime. For all we know it could be hiding at the bottom of a well somewhere in rural PA.
Concerning taking a memento from the crime scene I am sure it wouldn't be the knife.
Look how meticulous BK was in not leaving any digital evidence to connect him to the victims. I do not know what this means exactly, but I do not believe he did not see their social media. No connection because he apparently did not engage with them.
And no blood in his car or apartment means he planned it in my opinion. Unplanned with no plastic down would be impossible to not leave trace blood.
So a killer this focused on these evidence details would not hang onto his knife, would not keep anything in his car or apartment that links to the crime scene.
He had the trophy he longed for I am sure. Front page news and countless photos and news and social media coverage. Guessing games in the media about who did it? Where is the knife? How did the killer get in? House targeted or victims targeted?
He was able to sit back and revel in his knowledge of the case, he knew more than the police, probally felt some comeuppance for being rejected from LE programs.
Kicked out of...
His HS vocational LE program
Not getting an internship with Moscow police (I believe he did not get selected for that)
Kicked out of his PHD program that focused on criminals. The wheels were turning on kicking him out of his teaching PHD program before he committed multiple homicides.
2 Cents
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