The sentence is from the book Harry Potter and the Philosopher’s Stone. I can probably get the meaning of the sentence, but the grammar really confuses me, especially the usage of the word as. I could understand if the sentence was written as: It's they that should be sorry! More context:

"Sorry?" barked Hagrid, turning to stare at the Dursleys, who shrank back into the shadows. "It's them as should be sorry! ... ...

Can someone help to explain that sentence structure and grammar point? Thanks!