Symbolism, Imagery, Allegory
Todd describes the Spackle: "he's tall and thin like in the vids, white skin, long fingers and arms, the mouth midface where it ain't sposed to be, the ear flaps down by the jaw, eyes blacker than swamp stones, lichen and moss growing where clothes should be" (25.8). Sounds like a mystical, woodland alien, right? And that's exactly what it is. But more than being mystical, the Spackle is innocent—and that's his place in the story.
The Spackle is the ultimate victim. And this particular one that Todd meets and kills represents the whole race. Todd's people saw that they were different from themselves and destroyed them. Oh, after settling in their land. Todd's mother explains in her diary: "They're very sweet creachers. Different and maybe primitive […] but I don't agree with some of the thinking that the Spackle are animals rather than intelligent beings" (38.31). The Spackle clearly represent racism, oppression, and violence against those that are thought to be different. They are 'the Other.'
The scene where Todd kills the Spackle is pretty heart wrenching, and the moment where he realizes "he's weaker than me" (25.44) is a real turning point. Todd attacks based on what he's been told about the Spackle, but when he kills it, it's "Noise is filled with pain and bafflement and fear" (25.57). It's only after that Todd realizes what he's done—killed a totally innocent creature—and comes to see how his knowledge can't be trusted.
The Spackle represents how vulnerable the innocent are, including Todd, and how corrupt the big, ugly world can be. Because of the corruption of New World, Todd is tricked into killing an innocent creature, and when he does, his own innocence dies a bit, too.