Wearing shorts, the double-amputee removed his prosthetic limbs at the request of his lawyer Barry Roux as he made a final plea for Pistorius, who faces 15 years in jail.
The Paralympic athlete held onto wooden benches for support as he hobbled in front of the judge, and appeared in distress as a cushion was provided for him to rest on.
Roux and state lawyer Gerrie Nel set out their concluding arguments at the High Court in Pretoria, three years after Steenkamp's death.
"It is three o'clock in the morning, it is dark, he is on his stumps," Roux said, stressing his client's vulnerability.
More From This Section
"His balance is seriously compromised and... He would not be able to defend himself. He was anxious, he was frightened.
"His perception that he and the deceased were in danger was fortified by finding the open bathroom window. He believed the person in the toilet was an intruder and deceased was at the time in the bed," Roux said.
The standard jail term for murder in South Africa is 15 years, but Pistorius's sentence may be reduced due to the year he has already spent in prison and mitigating factors, including his disability.
Roux urged judge Thokozile Masipa to "entertain the correct facts and not to be drowned by the many perceptions" whirling around the case that attracted years of intense public scrutiny.
"The accused has lost everything. He can never ever resume his career," Roux said.
State lawyer Gerrie Nel started his arguments to push for a severe penalty.
"He knew there was someone behind the door," he said.
"Using a lethal weapon, a loaded firearm, the accused fired not one but four shots to the toilet door," Nel said. "He failed to provide any acceptable version for his conduct."
Pistorius was originally convicted of culpable homicide - the equivalent of manslaughter - until the appeal court upgraded his crime to murder.