He says he perceived a threat, however great he makes that perception out to be, what he is not claiming is that he saw a real threat and that is the key.
We have laws against killing for a reason, to prevent people from killing people. SA laws regarding self defense are not the same as US laws. In SA you are not permitted to use deadly force against a person that is not demonstrating an immediate threat to you or others, that is the law. If an armed theif is stealing your car on your property you cannot shoot at him, you have a responsibility to stay in your home and allow the theif to live. The value to human life is easy to understand from the SA law's intentions, the value of your car that he is stealing or anything else cannot be valued higher than a human's life, including the thief's life.
So OP violated the law. He did not visualize anyone that was immediately intend in causing him great harm or death. Instead he fired four shots, not just one by accident or whatever, fired four shots through a door intending to murder whomever was behind that door. That is against the law. Although he killed Reeva and not an intruder doesnt change the fact that he intended to kill, he fired four shots attempting to kill, and he killed a person - that is murder.