Section 13 is very complicated, very deep, and very important in understanding Nietzsche. The focus is on a contrast between lambs and birds of prey, in order to understand the origin of the concept of "good" as born from ressentiment. It is quite natural that lambs may consider birds of prey to be evil, since they kill and carry off lambs. And from this, it may also be understandable that lambs consider everything unlike birds of prey--themselves, for instance--to be good.
While Nietzsche accepts these conclusions as understandable, he denies that they can be used to reproach or condemn birds of prey for killing lambs. It would be as absurd to ask a bird of prey not to kill as it would be to ask a lamb to kill. Killing is an expression of strength, and it is only through a misunderstanding caused by language that we manage to see the bird of prey as somehow distinct from its expression of strength.
To illustrate his point, Nietzsche takes as an example the sentence "lightning flashes." Grammar would lead us to conclude that there is a subject--"lightning"--and a predicate--"flashes." But what is the lightning if not the flash? Nietzsche argues that grammar, and only grammar, has led us to think of actions in terms of subjects and predicates. In reality, he suggests, "'the doer' is merely a fiction added to the deed--the deed is everything."
Grammar has thus led us to think of a bird of prey as somehow separate from its expressions of strength, and thereby free either to kill or not to kill. On the contrary, Nietzsche suggests, the bird of prey is the strength is the killing. The lamb's morality is in no position to hold the bird of prey accountable for killing: that would be equivalent to blaming it for existing.
When slave morality lauds its conception of "good," praising all those who do not kill, hurt, or offend, it is essentially praising all those who are too powerless to cause any harm for not causing any harm. It interprets the inaction resulting from impotence as a positive, meritorious deed, as enduring ills and leaving revenge to God. Slave morality depends on the belief in a subject (or a "soul") which is independent of its deeds, so that it can interpret its weakness as freedom, and its inaction as praiseworthy.
Section 14 is a rather over-the-top depiction of slave morality being forged in a sweaty, smelly hole full of hatred and muttering. It culminates with the claim that "justice" is an invention of slave morality made out as an ideal that masters brazenly disregard. Slave morality does not seek revenge, but waits for the "Judgment of God" that will restore justice.