1. As for the definition of backbiting, we advise you to read Question 714 (site: 756) to know the conditions of backbiting and their exceptions.
2. If the person in question is really hurting others, spoiling their lives and doing bad to them and boasting over doing it, there is no problem in telling others about him and his evil deeds; it is not considered to be backbiting.
3. Every evil act to others has its own specific penalty or punishment. If a person does bad to others and fails to seek repentance or others' forgiveness, he will get the punishment proportionate to the bad deed he has committed to others. It has been narrated that the Holy Prophet (pbuh) said: "Allah will deprive from the fragrance of Paradise one who hurts his neighbor. His abode shall be Hell and what terrible abode it is! And one who fails to fulfill the rights of a neighbor is not one of us. Jibreel has emphasized so often on the rights of the neighbors that I thought they shall be included among one's inheritors." 
Imam Ja'far as-Sadiq (a.s.) says: "One who relates something about a believer to expose his defects and insults him so that he falls in the estimation of people will be deprived of Allah's friendship and guardianship. Allah will push him under the guardianship of Shaitan." 
Imam Ja'far as-Sadiq (a.s.) says: "…whoever causes a believer's property to perish or damages his reputation, it is obligatory on him to seek the other person's forgiveness…" 
For further information, see Index: "Backbiting People Who Do Not Offer Prayers", Question 714 (site: 756) .
 - Ameli, Shaykh Hurr, Wasail al-Shia, vol.12, pg. 127, Aalulbayt Institute, Qom, 1409 A.H.
 - Fattal Neishabouri, Muhammad bin Hasan, Rawzatul Wa'ezin, vol.2, pg.387 and 388, Razi Publictions, Qom; Al-Amali, Shaykh Saduq, pg.486, Islamic Library Publications, 1362 (Iranian calendar).
 - Tamimi Maghrebi, Nu'aman bin Muhammad, Da'aemul Islam, vol.2, pg.485, hadith 1731, Darul Ma'aref, Egypt, 1385 A.H.