"My lady, he knew it was a human being in the toilet. His intention was to kill a human being," prosecutor Gerrie Nel said in answering the defence's final arguments.
"[if Pistorius] shot into cubicle well-knowing there is a human being in there then he is guilty of murder. My lady, if someone shoots to kill then there must be consequences."
Because Pistorius had said he wanted to shoot an intruder at his Pretoria home on February 14 last year, and not his girlfriend, did not change the fact that it was murder, said Nel.
Pistorius is on trial for the murder of Steenkamp.
He shot and killed her through a locked toilet door at his home. He alleged he mistook her for an intruder. The State contends that he shot her after an argument.
Nel said the defences provided by his defence was "so mutually exclusive that they are mutually destructive".
Pistorius is also charged with three contraventions of the Firearms Control Act, one of illegal possession of ammunition and two of discharging a firearm in public. He has pleaded not guilty to these charges as well.
The case was postponed to September 11.