You need to look at Chapter 6 and 7 to answer this question. Re-reading this chapter reveals that the Court of the Terrenon is associated with darkness from the first time Ged hears the name from the itinerant sorceror that tries to persuade him to go there. Although Ged is initially relieved to find himself within the protection of the Terrenon (but note he does not know what has given him this protection from the Shadow), when he learns of the reality of the Terrenon he is clearly overwhelmed by its power but also its evil:

And his breath caught in his throat, and a sickness came over him for a moment. This was the foundingstone of the tower. This was the central place, and it was cold, bitter cold; nothing could ever warm the little room. This was  avery ancient thing: an old and terrible spirit was prisoned in that block of stone.

Therefore Ged recognises the power of the "evil spirit" that is enchained in that stone and the folly of dealing with it, even though it offers the potential of giving him what he needs to successfully fight the Shadow. Note that this is another section of the novel where Ged is tempted to do what he knows is wrong in order to gain what he needs to beat his Shadow - we see this again with the Dragons of Pendor, when the big dragon offers him the name of his shadow, but Ged sticks to his promise to the fishermen.