Another perspective: Indio was the young woman's pathologically possessive and dangerously jealous brother.
In the flashback, he had shot a young Douglas Mortimer who was the lover or husband of Indio's sister. Mortimer miraculously survived Indio's attempted murder. In the disturbing rape scene, Indio's innocent sister and moral opposite could not bring herself to shoot her abusing brother. So, rather than having to live through the trauma of losing her lover, being dehumanised by an incestuous rape thus scarred for life, she shot herself.
When the man with no name asked Colonel Mortimer if he were ever young, he cryptically spoke of something that had happened which made life very precious to him. That was falling in love with Indio's sister. After surviving Indio's bullets and finding out what had happened, he was obviously on his trail but was thwarted by Indio ending up in prison. When Indio had escaped from prison and 'no name' & Colonel Mortimer saw the 'wanted' poster, the former was looking at the $10 000 reward money while the latter was alerted that Indio was at large thus reviving the trail.
In the memorable closing scenes, Indio was shocked to hear Douglas Mortimer revealing who he was and surviving the bullets he had riddled him with in that disturbing flashback. In the powerful scene when Colonel Mortimer shot Indio in the duel, he symbolically reclaimed the watch he had gifted the girl from Indio's corpse.