That gets me thinking, though... why would it matter if they were a criminal or not? They'd already be dead from having their skulls removed!
Mhm but we were originally addressing your question of why he would possibly be interested in skulls. He's a pathologist. So he has access to cadavers. But why would he take them home? What possible reason? The only thing I can think is either trophies (but he incinorates them) or...for some kind of weird research? Unless he gives them to Landru. Dogs like a nice (femur) bone....
I can just see Werner or Rueb gettin' down wid da homies and sayin' "This is yo momma Hans" LOL biggin it up large - The Klopeks: da Mayfield Place Massive aaaaaaait easy nooooow.