Eko's finals words - explained. assuming that the nemesis and the monster are the same The monster has been judging people to find the perfect person for the loophole - the plan to kill Jacob. He needed someone who was special, or people would believe he was special. Judged the pilot, Locke, Eko, etc. The monster wanted Eko as it's 'human vessel'. It needs the perfect person/body. It was going to use Eko (and Eko, by the end, understands these plans). But Eko ticked off the monster/nemesis/Yemi. He says he doesnt seek forgiveness for the life he has lead. So the monster kills him. So, Eko tells Locke before he dies "you're next". He refers to the monster's next target, the next possible human who Nemesis can manipulate into a position of power, then kill and take control over the body, and then manipulate others into killing Jacob.
The monster/Nemesis has been plotting this for a veeeery long time.