What is SIN? 

Is it some action or thought that goes against a religion, moral or natural law? Is it something you are punished for? Is it a serious mistake or disobedience that you make against the will of GOD?

These are all true, valid and legit. 

But! What about those harms that you have done to yourself? The wrongs you have committed to your own self? Do they count as sins?

I'm not here to prove which is the worst sin one can do. But I'm here to tell you about the worst sin someone can do to themselves. The sins that affect others are the ones that might be forgiven by either GOD or the person whom you've hurt. But the sins you've committed to yourself, these are the worst ones, the ones that keep you up at night, the ones that never let you at peace.

The worst sin, in my opinion is: 

"To do wrong and never regret about it."

Throughout my life, I always thought that I'm not responsible for the suffering of others. I hurt people and never cared about their hearts. And that is my worst sin, not because I had hurt THEM, but because I had betrayed MYSELF. I had made myself the most miserable being, the one GOD doesn't even hate or dislike, but the ones that GOD has completely forsaken. 

There's a phrase from Surah Al-Baqarah (Verse 18) of the Holy Quran. It translates to :

"Deaf, dumb, and blind — so they will not return [to the right path]."
[Al-Quran / Surah Al-Baqarah (Verse 18)] 

Now one might ask why would Allah completely forsake such people?

Because GOD can forgive you for the sins you have done to HIM, but not for the sins you have done to others. And the sins you have done to others are actually the ones you have done to yourself. You have made your heart incapable of feeling regret, regret for the wrongs that you have done. And you no longer seek forgiveness for that, as you haven't accepted your sins. And this, dear reader is the worst sin...

TO NOT HAVE A HEART, a heart that can feel, cry and ask forgiveness. And that, that is your WORST SIN.

Self-Betrayal is the biggest betrayal of all.

As Fyodor Dostoevsky has once said:

"Your worst sin is that you've destroyed and betrayed yourself for nothing."
[Fyodor Dostoevsky - Crime and Punishment] 

I don't think people ever realize the sins they have committed to themselves or to others. They might realize it late in life. They fool themselves and keep their hearts shut, dead and incapable of love. For them love is not important, as their hearts are completely incapable of loving anyone, including themselves.

In my eyes, they are the worst sinners. But again, who am I to judge?