There is no harder punishment than when the servent has a cruel heart and moves away from Allah . Hellfire was made to melt such cruel hearts. The most remote heart from Allah is the cruel one. If the heart was cruel, the eye will never shed tears. Cruelty of the heart is caused by increasing four things over and above ones needs: excessive food, sleep, talk and socialising. When the body is sick, food and drink is no use to it. Likewise when the heart is ill because of desires , advice is useless...... Whoever wants to purify his heart must prefer Allah over and above his own desires.