Evil Genius FAQ

Freezer
  This room is where you keep your dead enemies.  It generates lots of heat if 
  an agent finds it with corpses inside, so it's a great candidate for stashing
  way in the back of your base.  In terms of size, if you use freezer racks (
  you should), a 8x4 room should be adequate for quite a long time.

  This room is unlocked when you kill an agent.  Kill one of the ones that start
  on your island, build the freezer, then kill the rest.

  Why build a freezer?  Dead bodies sap the loyalty of your minions who walk 
  past them and they'll desert if surrounded by piles of them.  The freezer 
  DOES decrease the decay rate (bodies take longer to poof), but your minions 
  won't lose loyalty for bodies in it.  Stashing bodies outside would be fine
  if agents didn't keep running across them.
  
  Freezer racks - lets you store 6 bodies in a 3x1 space.  In an 8x4 room, you
  can place these back-to-back and stick in 5.