Although injury makes an indirect appearance near the beginning of the poem, things really get gory toward the end, when the stepsisters are brought to gruesome justice. Many of these bloody events are told in a really casual, almost nondescript kind of way, which in a way makes them even more startling. After all, if somebody says something really shocking in an offhand way, don't you kind of do a double take?
Line 18: This is the only real instance of injury in the beginning of the poem, and it's kind of a subtle reference. We're supposed to think that the charwoman was injured in the bus crash and collected on insurance money because of it. In this case, the injury is actually the thing that brings about the woman's good fortune. In the rest of the poem, the blood and guts will serve a nearly opposite purpose—as justice for evil-doers.
Lines 81-85: First gross-out alert. The first stepsister wants so desperately to marry the prince that she slices off her toe. Notice the nonchalant way this is described: "simply sliced it off" (83). Egad. Of course this ploy doesn't work, and the prince is tipped off to the deception by noticing the "blood pouring forth." This is some pretty graphic imagery right here, which highlights how grim the real Grimm tale was (pun intended).
Lines 88-89: This image is a repeat of the one just above it, this time with a portion of the second stepsister's heel instead of a toe.
Lines 95-99: In this final and particularly violent instance of gore, the stepsisters have their eyes pecked out by the white dove. This is, you know, gross. It's also straight-up revenge. And notice the particularly chilling simile Sexton uses in line 99: "hollow spots like soup spoons." Again, it's a nonchalant comparison—soup spoons are pretty ordinary objects, but empty eye sockets are anything but. Yikes!