Worst sin is the one taken lightly
Nahj al-Balāgha|Volume 1|Book 3|Chapter 477
٤٧٧. وَقَالَ (عليه السلام): أَشَدُّ الذُّنُوبِ مَا اسْتَخَفَّ بِهِ صَاحِبُهُ.
477. Amīr al-mu’minīn, peace be upon him, said: The worst sin is that which the committer takes lightly.