there was a prophecy made of which the content was that a boy born at the end of july, whose parents has defied voldemort thrice and to whom voldemort will mark him his equal(he'll have powers not known by voldemort (which is love)), will either kill voldemort or voldemort will kill him. there were 2 boys who were neville and harry. so, if harry wasnt born, voldemort would've marked neville his equal, and neville would've become the chosen one.
though there is a possibility that neville might have died or his parents wouldnt have died for him...so its not 100% right to say that neville would've killed voldemort