AJ Ross is suing the pet store retailer over the 2020 death of her poodle, Kobe -- which happened after she dropped off her pooch for a nail clipping at a PetSmart in Pittsburgh, PA.

In the lawsuit, she lays blame directly at the feet of the store's employees -- and adds, the store's own surveillance video is the most compelling evidence supporting her case.

The footage is tough to watch. You see Ross drop the dog off with the staff, having already placed a muzzle on its mouth, and then leave. The workers then take it to the back and lift it onto a table.

That's where things get wild ... it appears the employees put 2 different harnesses around Kobe's neck -- one from the top and one from the bottom -- seemingly to keep the dog in place while they try to work on its paws.

Kobe thrashes around, and they eventually tip the poodle over on its side for easier access ... at least that's what Ross claims in her suit.

Soon enough, the dog goes limp and the workers try waking it up, to no avail. Ross says when she returned a mere 10 minutes later, she discovered her dog was dead -- and she believes the way the PetSmart employees harnessed Kobe during the nail clipping caused the dog to lose consciousness and die.

According to documents, Ashley claims PetSmart has allowed this to happen to other pets over the years -- upwards of 47 have allegedly been killed by PetSmart groomers since 2009 -- and now she wants damages.

A rep for PetSmart tells TMZ ... "All of us are heartbroken by and truly sorry for the loss of Kobe. Following this terrible accident in 2020, we immediately launched an internal investigation and cooperated with the authorities involved. In response, we terminated the associates who violated our pet safety policies and did not meet our high standards of care."