Why do you think the Warden scratches Mr. Sir?

The Warden in Holes is a truly fearsome individual. A nasty, egomaniacal control freak and bully, she simply will not tolerate any challenge to her authority, however minor. A particularly stark illustration of this comes in chapter 20 when Mr. Sir brings Stanley to the Warden's office. Stanley has been accused of stealing Mr. Sir's sack of sunflower seeds. In front of the Warden, Stanley admits to stealing the seeds, but Mr. Sir isn't so sure; he thinks that Stanley's just covering for someone else.
It's at that point that the Warden asks Stanley to go fetch her makeup case. Inside is a little bottle of dark-red nail varnish, which the Warden proceeds to apply to her fingernails. The varnish contains a special ingredient: rattlesnake venom, which is pretty harmless when dry, but fatal when wet. After creepily stroking Stanley's face, the Warden slaps Mr. Sir right across the chops, leaving three dark scratches across his face. He writhes on the floor in agonizing pain, but fortunately for him, he won't die, as thankfully, the nail varnish appears to have dried. The Warden hits Mr. Sir because she feels he was wasting her time over something so incredibly trivial as the petty theft of a bag of sunflower seeds.