One gets great rewards if he abandons haram. It elevates man to the degree of taqwa. If a person commits haram, some harms and losses are in question; for instance, his heart blackens, his conscience becomes rusty, his belief weakens, his peace and joy disappear, his feeling of obtaining pleasure from worshipping disappears. Besides, a severe torture in the hereafter is in question.  

To regard something that is definitely haram as halal makes a person, Allah forbid, exit Islam.