The key word in the first sentence is "fights." When one fight, one often justifies one's actions by what it does to one's opponents. People who consider killing to be wrong are often willing to take a life in self defense.
In practice, it is often very hard to stop undertaking these actions when the monster is gone. Consider the case of the warrior who has to come back to live in a society where the instincts that kept him alive for years now have disastrously unacceptable outcomes.
There is also the concern of the monster's perspective. Often the mere fact that you are fighting against them can make you a monster in their eyes. Then you have to consider all of these issues from their perspective. Suddenly they may be willing to kill in self defense when they otherwise thought killing was wrong.
The abyss is a more difficult one to capture in words. If given a choice, I prefer to simply stare at the phrasing you already have: "If you gaze for long into an abyss, the abyss gazes also into you." At the very least, there is certainly a reflexive property to this action. You are staring at the abyss, presumably with the intent to understand it. This means you must be forming an image of the abyss in your mind. When it comes to concepts of emptyness, the image of emptiness is often indistinguishable from the emptyness itself. As a boring mathematical example, it is stated that "there is only one empty set." So if you try to observe an empty set, and form an image of it in your mind, that image must, itself, be the empty set. Contrast that with trying to observe a person. In that case, the image of a person is not the same thing as the person itself.