Prophet (peace be upon him and his progeny) said:
اِذا سَرَّتْكَ حَسَنَتُكَ وَ سائَتْكَ سَيِّئَتُكَ فَاَنْتَ مُؤْمِنٌ
When your good deed makes you happy and your bad deed disturbs you, you are a believer.1
Islam says that all men are born with a pure primordial nature, which believes in and loves to do good. Taints and sins begin to gradually affect his spirit and soul and change his nature and in time turn it totally. When you see that a man is interested in goodness and hates bad deeds, you know that the spirit of belief and first pure nature still exist in him.
What a miserable lot are those who, instead of being disturbed by their bad deeds, take pleasure in them , or who are resentful of their deeds of self-sacrifice, righteousness, forgiveness and justice. They are the unbelievers.