Who is The Mutant Vampires from I Am Legend and what is their importance?

Robert discovers that the vampire germ eventually has mutated, resulting in a race of vampires who have overcome the psychological effects of the disease and who have learned how to survive in the daylight for brief periods. These vampires hate and fear Robert for killing them off, and are determined to capture and kill him. They send one of their members, Ruth, to spy on Robert and learn what he has learned. Their goal is to increase their numbers and to reclaim society with violence. They also regard the non-mutants as inferior and have embarked on a campaign to execute them on sight. Although their orders are to bring Robert back alive, they break into his house with guns drawn, and Robert responds with gunfire. The mutants shoot back, and Robert is seriously wounded.