Unfortunately, if a person goes out to deliberately kill another ( murder ) and kills the wrong Person by mistake; that then becomes Manslaughter. They did not intentionally kill that person, it was by mistake ( Manslaughter ). It could not be proved the He KNEW it was Reeva Steenkamp in the Toilet, hence it could not be proved that it was HER He deliberately killed. He sure as Hell meant to kill whoever was in there though.

http://www.ibtimes.com/what-are-expanding-bullets-dum-dums-used-oscar-pistorius-shoot-reeva-steenkam...