The difference is...... IF OP shot KNOWING Reeva was behind the door.. then I for one would want him found guilty of the highest degree of murder possible in SA.. sentenced to LWOP.
IF he shot thinking it was an intruder, then the crime (if any) is open to debate.. depends on details of law in SA. Personally I do not have a problem with shooting to kill an intruder. If OP genuinely thought he was doing just that, then it becomes a case of mistaken identity.. an accident even, but I imagine some sort of lesser crime such as manslaughter (or the SA equivalent)
BBM:
I disagree......he didn't know what was behind that door.....what if his dog climbed the ladder...my cat even?....sheeesh. ......trigger happy should have shouted something more than " get out of my house"....oooooo scarey.