With all due respect. Serial killers do this. A lot. It’s not serial killer 101 at all. For example..
without going into too much detail- a case that’s quite famous, serial
grape and murder. One person was caught but while in prison, after many years, finally admitted he had not only had an accomplice but that the other man was the ring leader. This person had been free for a long time. Every time they offended, on the way back, they would listen to a certain song on cassette in the car. That cassette was found under the other man’s floorboards in his home when they searched. Despite almost getting away with such heinous crimes, he couldn’t part with it.
Almost every medical serial killer I’ve read about has kept things. Many of them still kept them despite knowing eyes were on them and even police had begun investigating. One of my faves is a book of poisons with all the corners turned down in the pages of the poisons she used

clever woman, graduated early from college, mum, socially skilled. Killed lots of patients for kicks.