Benali scrambled the ball behind for a corner but most of the lowest Premiership crowd at the Stadium of Light this season had long gone such was their frustration and increasing acceptance of relegation.
Southampton climbed back into the top six but manager Gordon Strachan had sympathy for his opposite number Howard Wilkinson who faces a mammoth task to stem the tide.
"I was delighted with the performance,'' said Strachan.
"I know how Howard feels because we have been there. We can relax and I think that was the difference.
"There weren't many chances but we had a spell towards the end of the first half when we could have scored.''
Rory Delap missed a great chance in the tenth minute when he headed a Paul Telfer centre over the bar from close range and in the 29th minute Fabrice Fernandes struck the post from a free kick.
Beattie fired over the bar immediately after the break but the in-form striker kept his cool shortly afterwards to leave Sunderland without a win in the last seven Premiership matches.