Predicate Logic - negating the conclusion to prove logical consequence?
The method works in the same way as proofs by contradiction. Suppose you have a set of assumptions $A$ and you want to prove that formula $\phi$ is a logical consequence of the formulas in $A$. One of the ways to do this is as follows.


Put the negation of $\phi$ in your assumptions. So, now $A' = A\cup\{\neg\phi\}$
Show that $A'$ is inconsistent. Here, this means to consecutively apply the resolution step on (appropriately chosen) formulas of $A'$, until a contradiction $(T\to F)$ is reached.


Classical logic tells you that, if (a) the initial set of assumptions $A$ is consistent and (b) $A\cup\{\neg\phi\}$ is inconsistent, then $A\cup\{\phi\}$ is consistent and you can say that $\phi$ is a logical consequence of $A$. So, if steps (1) and (2) succeed, then $\phi$ is a logical consequence of $A$.

If you don't negate the goal when you put it in the set of assumptions and  if you manage to show that $A\cup\{\phi\}$ is inconsistent, then $A\cup\{\neg\phi\}$ is consistent and therefore $\neg\phi$ is a logical consequence of $A$.