Alberto_Italiano

Indeed, it is Nietzsche. Cannot recall which book.

But first, my two cents on being bullied. At school once it happened to me, I was 14, and I was obviously targeted - for months.
What worked for me, and solved it, was to finally accept the challenge: instead than recoiling, face the guy squarely and say: "ok you wanna a fight I'm gonna fight this time. You start".
In my case worked and, to my own bewilderment, ended the bullying and I even hadn't to fight go figure!

I am not saying anyone should rely on this - there are bullies that may be far more of an issue than that. However, it may indicate that a bully goes on bullying only as long as he gets away with it for free, or with minimal losses.
Also in another instance I found out that the mere verbal confrontation (never abusive: meet him on tough but not verbally abusive grounds, or you play his playbook) ended it.
But of course those probably weren't the callous bullies.
However...

As for Nietzsche, at face value the sentence says what it says, and it is true. But the second part (the one about the abyss) seems to suggest a subtler meaning: if you deal long enough with daemons, you may not become a daemon, but it will leave a mark. Nobody copes with evil with impunity - or that wasn't evil.

Everything conjures for our own Enlightenment, and evil most than anything else because it is so real.
How much real? Enough to kill us! And when it's truly evil, it won't use its hands.

It is as if it is saying: ok, you think you're a tough guy - show me you are one, let's see how much you like this. And it produces something that is, indeed, a challenge.

If you like killing dragons, you must kill the ones that are hard to kill. That's when the dragon "stares back into you". A stare you will never forget. And that was the real deal, the real "dragon".
It leaves you fighting with yourself.