Red Lantern Corps
After Atros killed his own people in order to harness the red element, he was forever changed. His heart stopped beating and his blood was expelled from his body and replaced with burning plasma. He felt constant rage as long as he wore the ring. He soon found other aliens filled with rage in their hearts and they were inducted into the Corps.
Killing Abin Sur
During a mission, Atrocitus encountered the Green Lantern known as Abin Sur. He attacked his ship and they battled. Eventually, Atrocitus landed a blow that would eventually lead to Abin's death. He sent him crashing to Earth as Atrocitus flew away victorious.
At some point in the future Atrocitus had been killed and his spirit was absorbed into Butcher