Just did some testing on a copied hood.  Everytime an apartment sim would walkby that I haven't modified, I killed it.  I ended up with 21 kills, which got rid of most of them.  (The walkbys became a maze of tombstones  

 ) Then I jotted down the walkers.
Out of 50 walkbys:
15 were original sims.
7 were then killed.  (apart. sims)
the other 28 where walkers which consisted of just 5 sims.
I'm assuming the game isn't using pure random function to decide who goes.  It appears to favor the newer sims a lot more.