Justice Lawrence Wargrave was a former judge. He retired, and invited the rest of the ten to soldier island. Having a romantic imagination, he always wanted to murder people who were guilty. So, he pretends to be a victim of the murderous U.N.Owen by creating a plot with doctor Armstrong to pretend to die. He ends up killing Armstrong and letting Blore, Lombard, and Claythorne finish off the job. To show that he was the real "U.N.Owen", Agatha Christie never shows the story from his point of view.
Wargrave pretended to be the one who was giving the rest of the group suggestions for who the murderer was. By doing this, he eventually makes Claythorne think that Lombard is the murderer, and vice versa. He was also the only one of the "little soldiers" who was not truly guilty of crime, but rather justly doing his job.
Wargrave hired the Rogerses and sent all of the notes to the guests. He put cyanide in Marston's liquor, put a lethal dose of chloral hydrate in Mrs. Roger's brandy, hit Macarthur in the head, split Roger's head open with an axe, stabbed Brent with the hypodermic syringe, pretended to be shot in the head (the plot with Armstrong), pushed Armstrong off of a cliff into the ocean, dropped the clock onto Blore's head, and let Claythorne shoot Lombard. He finally tests Claytthorne by seeing if she would hang herself. She did. He then wrote his confession, threw it into the sea, and shot himself dramatically.
Wargrave thought that the guilty should pay. Pay they did. Wargrave thought that the innocent should not be harmed. Harmed they were not. In conclusion, Justice Lawrence Wargrave was the murderous lunatic who was behind the scenes, pulling the strings, proclaiming himself as U.N.Owen.
Thank you, thank you very much.