In the novel from which the movie was taken, Marty shoots Ethan who is attempting to kill Debbie (different names in the book).
The screenplay did share similarities with a real-life incident that took place at Fort Parker in East Texas on May 19th 1836. At 10 o’clock in the morning, a tribe of Comanches killed and scalped five men, including Ranger captain Silas Parker, tortured and raped several women, and kidnapped 17 year-old Rachel Parker Plummer, and her toddler son, her 9 year-old sister Cynthia Ann Parker, her 7 year-old brother John Richard, and a girl called Elizabeth Kellogg. Rachel Plummer wrote down the events of the raid and her capture and told the story of her 13 month ordeal in detail.
On the 6th day of their capture, the hostages were divided up. Elizabeth Kellogg was traded with a band of Kichai Indians, Cynthia Ann and James went to a band of middle Comanches, and Rachel and her son to another tribe. Rachel was eventually purchased by a group of Comancheros (who wouldn’t acquire that actual name for another five years) in August 1837. They were working for a wealthy Sante Fe couple, William and Mary Donohoe who told them to pay any price for white women. The couple took Rachel in, took care of her and promised to return her home. She eventually returned and gave birth to another son before her premature death.
Cynthia Ann was rescued by the Texas Rangers over two decades later in the Battle of Pease River in late 1860, after living as the Comanche wife of warrior Peta Nocona ,and the mother of three Native American children (two sons, one of whom was the famous blue eyed Comanche chief Quanah Parker, and a daughter).She had great difficulty adjusting to white society upon her return and died in 1870.
http://www.imdb.com/title/tt0049730/
I do not think so, I though a girl he thought was Debbie, was actually a Comanche and shot him. Marty (Martin) finds Debbie outside the camp. Ethan (Amos) was going to shoot the girl but had changed his mind, which gave the Comanche girl the opportunity to kill him.
Ford did change the ending, but not the most important part, that Ethan still had pity and compassion.