Video: Man snatches child in Walmart, holds her hostage and then...

OKLAHOMA CITY (CNN) -- Imagine going shopping with your children at a local supermarket and then suddenly having your child plucked right out of the shopping cart and held hostage.

Surveillance video captures a man holding a 2-year-old girl hostage with a knife at grocery store.

That's exactly what dramatic video of a deadly stand-off shows. It all unfolded in the middle of the day at an Oklahoma City Walmart.

Store surveillance video shows a mom wearing a white shirt shopping with her two daughters; seconds later while the mother's back is turned, this stranger reaches in and grabs her 2-year-old little girl from the shopping cart. Her stunned 12-year-old sister watches in horror.

The man, Sammie Wallace, hands a cell phone to the girl's mother and demands she call a police officer he knows in Dallas, Texas, all while holding a knife to the little girl's stomach, according to police.

Frightened shoppers frantically call 911.

"He took this woman's kid and we need a police here immediately," one shopper said.

"There's someone with a knife holding a little girl hostage," another shopper told 911.

Terry Parker, a local pastor, watched the dramatic scene unfold and heroically blocked Wallace with his cart until the police arrived.

"If that gentleman was to come in my direction, he had to go through me or my cart if he wanted to do that," he said.

Moments later SWAT teams evacuated the building and two police officers began to negotiate, but it doesn't work.

Police said Wallace was not responding coherently, that he was acting unstable and making unreasonable demands.

Afraid for the child's safety, police moved the mother and her older daughter to the front of the store.

Wallace starts counting down from sixty, threatening to harm the child, moving the knife towards the little girl's neck as she starts to squirm.

Then while one negotiator distracts Wallace, another goes up and shoots him point-blank in the head.

The officer immediately grabbed the little girl, who was unharmed.

Wallace was killed instantly.

The Midwest City Police Department and Oklahoma County district attorney David Prater have determined the officer who opened fire acted properly.

"He knew if he was going to take the shot he had only one chance. He made sure he was at point-blank range so there was no ability to harm that child," Midwest City Police Chief Brandon Clabes said.