Sin or Rebellion?

Baba Najam Ahsan khalifa of HakeemulUmmat (may Allah have mercy on them) said;

“The definition of a sin is that; if one commits it he feels remorseful and questions himself, ‘why did I do it?’
If one does not feel any regret after performing this action then it is not a sin but it becomes a rebellion (against Allah).”

Zikr-e-Ahsan 183