Because darkness blinds you. Thus it is not a strength, and if you use it as your ally, it will overcome you.
Is Satan a metaphor?
He would like you to believe that, wouldn't he? Wouldn't we all like to believe that? But to paraphrase and slightly modify the edgelord philosopher F.W. Nietzsche, you believe Satan doesn't exist, and what good are all believers to Satan?
Aren't the masses weak?
Are you weaker than them because you rejected society, or because you have a fundamental character flaw?
Don't enlightened people not believe in good and evil?
No, criminals, reprobates, libertines, and sorcerors, all being consumed by the abyss don't believe in good and evil, and they pay for it dearly even in this life. Does that sound enlightened?
Won't I become an overman if I cross the abyss?
No, the abyss never ends. That's the "joke". And it's not even remotely funny to anyone except psychopathic philosophers who are themselves drowning in the abyss.
Why should I serve others besides myself?
Because you should love your neighbor as yourself, but also on a more selfish level, if you serve only yourself, eventually you'll be all alone with no one to help you.
How do I become an overman?
You don't. But in the process you kill all of your emotions, lose sight of reality or even the ability to comprehend reality, and become less than human.
What's wrong with being-toward-death? It's authentic.
What's wrong with it is that Heidegger could have picked the truth as a thing that Being-there (Dasein) could orient itself toward, but instead he picked death.
Aren't all of our lives fundamentally worthless?
You think your life is worthless because your life is not impressive to righteous people, and you're lining up to be discarded into Hell.
Aren't the faithful stupid?
The faithful come in all stripes. Edgelords come in one flavor: intelligent, alienated, and resentful, giving a bitter antisocial taste that God spits out into the abyss until repentance comes, if it ever does.
I am a misanthrope. I don't care what you think.
You hate human beings far less than you hate yourself.
So what if we go to Hell? I'll just redefine it as something I like.
Hell gives the illusion that we can redefine a great many things and it will change reality, but you can't change the reality of the ground you stand on: quicksand with lava underneath.
A world without religion is a world of freedom isn't it?
Most of the 1.3 billion Catholics freely choose to believe their religion, and you should use your Free Will to make the right choice too.