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A pit bull owned by NFL star Trent Williams killed a neighbor's 3-year-old black lab -- and while the neighbor is PISSED -- Williams says it's no big deal.

Linda Flowers says her beloved dog, Sandy, was attacked and dragged through a fence by one of 9 pitbulls at Williams' Missouri City, TX home, ripping its body to shreds in the process.

Flowers tells TMZ Sports ... Sandy was docile and friendly and is clearly the victim in the situation. She believes Williams is running a dangerous pit bull mastiff kennel out of his backyard in a residential community with families and children.

Williams, however, told FOX 5 in D.C. he doesn't understand why the mauling is such a big deal ... saying it's "dogs doing what dogs do."

Williams claims his dogs did nothing wrong -- and it was Sandy who broke into his yard and attacked his dogs first.

We've learned Missouri City PD responded to the scene -- but kicked the case over to animal control, which is currently investigating.

No arrests have been made and no charges filed at this time.

31-year old Williams is a star for the Redskins -- the #4 overall pick in 2010 is a 7-time Pro Bowler and signed a 5-year, $66 million contract in 2015.