I have a pretty heavy suspicion that these aren't murder victims. If the only thing found was a small selection of mismatched bones unceremoniously stored in a box and left to an empty apartment, well... doesn't sound like a serial killing to me. Sounds really weird that a killer would have just taken random bones and then later abandoned them to such an easy location. Unless there was, of course, something more found with the bones the police doesn't want to publish yet.
I'd say a better explanation would be grave robbing. Two possibilities that come to mind are:
- the grave robber had some sort of superstitious, psychotic or otherwise mystic purpose for the bones. An attempt to cast a spell or curse? A way to cancel a supposed "curse" or bad luck the robber felt had something to do with those particular deceased? Some other, even more delusional belief? The bones were left behind because the grave robber views their focus fulfilled.
- the grave robber was a necrophiliac. They didn't have the chance to rob entire body but wanted to take trophies home anyway? Maybe the bones were left behind and hidden to that apartment as the robber was afraid of getting caught by someone if the stash was found. Or maybe the robber experienced a moment of remorse, decided to stop their habit and ditched the stash to resist the urge. This kind f behavior is very, very common to people with behavioral addictions such as kleptomania, exhibitionism and underwear/shoe/sock- stealing, sniffing etc.