Oscar Pistorius swore in court today that he believes he was shooting at an intruder when he gunned down girlfriend Reeva Steenkamp ... but prosecutors scoffed, saying he was hell-bent on killing her.

Pistorius' lawyer read his client's sworn affidavit to the judge, explaining that he believed an intruder was in his house on Valentine's Day and he felt vulnerable because he was in bed without his prosthetic legs ... and that's why he pumped a round of bullets into the bathroom door.

The prosecutor saw it differently ... he said Pistorius' motive was simple -- "I want to kill."  Prosecutor Gerrie Nel said the Olympian took the time to put his prosthetic legs on, walk 23 feet, and then he began shooting.

Pistorius claims he had no idea he was shooting at Steenkamp until he realized she was not in his bed.  He said, "It filled me with horror and fear."

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