What does is mean when you look into the abyss the abyss looks back at you?

B or C answer:
It means for instance, that if you fight evil, you should take care that you do not become evil yourself. This is actually the explanation for the line before this quote in Nietzsche's essay. The abyss looks back at you means that when you begin to know somthing that is fundamentally different from yourself, you take a piece of it with you and it changes you and vice versa.
A answer:
An aphorism is a demonstration of a profound idea that is not explicitly laid out by the aphorism. To "get" an aphorism, you have to adopt the perspective of the person writing the aphorism mapped out by the explicit situation in the aphorism. This aphorism is about the double-edged sword of free will. We are all, if we choose to be, free to care about whatever we want. Exercising this choice creates subjective moral meaning in life. However, sometimes our choices have unintended natural consequences: so there are limits on freedom. Caring about fighting monsters, in the first sentence of the complete version of this aphorism, creates the danger (unintended natural consequence) that one might turn into a monster oneself. Go find your own examples in literature and history where a person crusaded against some evil, but the cure was arguably worse than the disease. On the other hand, don't be too careful about what you choose to care about: what to care about has no answer, and things are meaningless until you give them meaning by choosing to care about them, and thinking about this too much is actually choosing to care about meaninglessness, which will rob a person of meaning. Extra credit: can we change morality and meaning and what we care about