Hakim al-Umma Shaykh Mawlana Ashraf Ali Thanawi (may Allah ahve mercy on him) said:

Committing a sin is a very wretched, ghastly and fatal thing.
It is essential to abstain from it completely.
That very moment when a slave in acting against his Lord’s command, is a very ominous and detesting. If he had the capabilities for differentiating between various feelings, he could have felt the darkening effect of sin.

Some wrong actions have the effect of causing the capability of performing good works in future to be taken away. This is a thing to be dreaded.

Another effect of sin is that the individuals under your command start disobeying your instructions.

The worst effect of committing sin is that due to the casual and careless attitude (of a sinner) a minor (saghirah) sin leads to a major (kabirah) sin and eventually to rejecting the truth (kufr).

Thus an individual should never become carefree after committing a sin. He should repent and make amendments.

Basa’ir-e-Hakeemul Ummat 532