The dog
The dog was the most amazing thing about this movie, all the things he did and he was very smart. I never understood, though, why he killed that little dog. At that point I though he will kill people later on but he didn't.
shareIt was forshadowing to show that when threatened the "embryo creatures" would resort to violence, then hide their deeds. When Victoria's life is threatened by her cellular aging she murders the prostitue and hides the body in much the same way Number One (the Doberman) did to the little dog.
My personal opinion is that it was to show that while the "embryo creatures" were of above average intelligence they lacked emotional attachment to other beings, thus they did not understand right from wrong.