I absolutely agree that firing through a door when you have no way of knowing who is on the other side -- and there is no visible sign there is an intruder in your residence -- should be considered manslaughter. I'm not so sure that is the case in some of the American states that have Stand Your Ground laws.
In those states a home owner or apartment tenant is presumed to have been legitimately defending themselves with deadly force if they say that's what they believed they were doing. They have the absolute right to kill anyone they believe has invaded their home. They don't have to prove they were in fear of their life or serious injury, they just have to state it. The burden is entirely on the prosecution to prove they are lying. Since fear is such a subjective emotion that can be very difficult to do.
Not to say that if this incident had happened in the U.S. Pistorius wouldn't have been prosecuted, but convicting him may have been difficult in SYG states. Where courts and juries recognize that, in protecting people's right to kill intruders, there will be mistakes made. That so long as the killer contends he killed in 'good faith' -- and the state can't
prove otherwise -- the defendants go free.
There have been many many cases in the U.S. that have left police, prosecutors and much of the public shaking their heads.