What topics related to the following quotes can I discuss in my essay on Lord of the Flies by William Golding? "You can feel as if you're not hunting, but being hunted, as if something's behind you all the time in the jungle." The rock struck Piggy a glancing blow from chin to knee; the conch exploded into a thousand white fragments and ceased to exist." "'Kill the beast! Cut his throat! Spill his blood!'"

You have a good start here with a selection of important passages. In your introduction, you might want to discuss how the narrative arc of the novel is reflected in the intensification of fear and violence from one quotation to the next. At first, the boys feel a formless fear of the primitive, reacting on a primal rather than rational level. The destruction of the conch represents a violent break with civilized values. The final call for murder completes the descent into primal terror and ensuing violence. 
For themes, you might start with talking about how the act of hunting, although intended to obtain food, also stimulates a sense of insecurity, and that efforts of the boys to make their lives more secure in terms of survival also expose them to the raw insecurity of primitive existence in the absence of a civilized safety net providing food and shelter.
For the second quotation you should look at how both the conch and Piggy represent the civilized, rational intellect, and how their destruction represents the disintegration of the superego.
The boys' final descent into murder of "the beast" portrays the boys trying to silence their fears by externalizing them and then killing a scapegoat. This reification of formless fear, however, seems to intensify fear and rage rather than removing it. Had the rescuers not arrived, it is likely that the boys would have descended into a downward spiral of violence and superstition.