Thrasymuchus was very hostile against Socrates in Book I. Every attempt that was made to prove Thrasymuchus wrong was badgered by bad comments of Socrates trying to manipulate him and the others. Socrates from the beginning on 336b was asking questions all the way to 347e building up his defense to Thrasymuchus statement that "justice is the interest of the stronger party" (Pg. 338 Para. C). Socrates disagrees with Thrasymuchus and he states, "Surely, then, no doctor, insofar as he is a doctor, seeks or orders what is advantageous to himself, but what is advantageous to his patient?" (Pg. 342 Para. D). Thrasymuchus and Socrates agreed to all these professions and what was considered a leader and what wasn't considered a leader. Socrates goes on with this, "… that a ship's captain or ruler won't seek and order to what is advantageous to himself, but what is advantageous to a sailor …" (Pg. 342 Para. E). Specifically there in those two statements Socrates has already shot down the idea of Thrasymuchus, but here is an example that Thrasymuchus gives to defend himself earlier in the reading:
And each makes laws to its own advantage. Democracy makes democratic laws, tyranny makes tyrannical laws, and so on with others. And they declare what they have made—what is to their own advantage—to be just for their subjects, and they punish anyone who goes against this as lawless and unjust. This, then, is what I say justice is, the same in all cities, the advantage of established rule. Since the established rule is surely stronger, anyone who reasons correctly will conclude that the just is the same everywhere, namely, the advantage of the stronger. (Pg. 338 Para. E)
At first you could easily agree with Thrasymuchus about his views and the stronger. Socrates fortunately pulls his idea apart and examines the different parts of it. To finally come to a conclusion that yes the stronger do rule over the weaker but not everything they rule on is for their own advantage, but the advantage of the weaker. That is why they are ruled, because they need a control.
In my opinion I agree with Socrates and Thrasymuchus at the same time. Back in their days there are many times where rulers took advantage of their power, and heavily exploiting the people and what there worth so they could live the good life.