The stick sharpened at both ends definitely alludes to the fact that Jack and his tribe are planning to hunt down, kill, and behead Ralph just as they had done to the sow they offered as a sacrifice to the Beast.
Of course, neither the twins nor Ralph know at this point about the sow’s head. It isn’t until later, when Ralph encounters the meatless, horrifying skull atop a sharpened stick, that he makes the connection with what Samneric told him as he hid.
Therefore, the stick sharpened at both ends serves as a cryptic warning of the dehumanizing, savage violence that Jack wishes to exercise on Ralph. Interpreted this way, one could certainly argue that the stick represents pure evil; however, I would suggest that the stick is representative of violence while the Lord of the Flies itself (sow’s head when Simon “talks” to it) symbolizes evil.