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The Prophet Muhammad (P.B.U.H) said:
when your good deed makes you happy and your bad deed disturbs you, you are a believer.
Nahj-ul-fasaha/P 41

Brief discription:
islam says that all men are born with a pure primordial nature, nature of belief and love to goodness. taints and sins may affect gradually on man's spirit and soul and change it and turn it totally. however, until man is interested in goodness and hates badness, the spirit of belief and first pure nature still exist in him.
miserable people who are not only disturbed by their bad deeds, but also glory in them and or when they do a self-sacrifice, righteousness, forgiveness and justice become regretful, they are unbelievers.

قال رسول الله صل الله عليه و آله

اذا سَرَّتْكَ حَسَنَتُكَ وَ سائَتْكَ سَيِّئَتُكَ فَاَنْتَ مُؤْمِنٌ

هر گاه کردار نيکت تو را شاد کرد، و عمل نادرستت آزارت داد، پس تو مومن هستي

نهج الفصاحه/صفحه41