The Paralympic champion shot dead Reeva by firing four shots through a bathroom door at his home in Pretoria, South Africa in the early hours of Valentine's Day last year.
In an interview with Australian SBS ONE show Dateline, Barry said: "He could have gone to the balcony and shouted for help knowing that somebody was there, he could have pressed the alarm system, all this would have been prevented.
"I think there was more to the whole story coming up to the actual shooting, the killing. If he's watching, he'll know what I'm talking about."
Pistorius could face a maximum jail term of 15 years when he is sentenced on October 13.