Maybe I should be easier on Archer...

From http://www.geocities.com/max3075/fate/ubw.htm

He was actually Emiya Shirou from the future. The artifact that summoned him was the pendant that Rin used to save his life. He really did become a hero that he wanted to! It was possible for him to be summoned from the future because the heroic spirits are beings separated from the timeline of our normal world. One thing that's different about his case is that he chose to become an heroic spirit himself, so he can help more people. Most of the other heroes are forced to become an heroic spirit because of their fame and power, but in Emiya's case it was out of his choice. He's been acting weird since the beginning of this story, trying to get at Shirou whenever he had a chance. The reason he wanted to kill him was because becoming a hero was very different from his ideal, and wanted to kill his existence beforehand. An hero can only save someone who he swore to protect and many sacrifices were made when he tried to do something. In other words, he was giving up. Through the fight with himself, he is shocked by the determination he had back then. He decides to keep on fighting once again. At the end, he tells Rin with a smile "I've got the answer. It's going to be all right Tohsaka. I'll do my best from now on too" He will keep on being summoned to fight for someone forever, but he's happy to do so because that's the choice he made.