She was the Deputy's Wife

In a small frontier town, Paul, an outlaw and horse thief, is captured and scheduled to hang. Two parties intervene simultaneously: Marshal Graham dispatches a telegram urging the local sheriff to delay the execution so Paul can be questioned about a missing white stallion, while Mabel, the wife of deputy Jonathan, arrives at the gallows with a hidden pistol, having fallen in love with the condemned man. The marshal's telegram succeeds in postponing the hanging, but it also opens a window for Mabel to act. She goes to the jail and frees Paul, but the jailer is killed in the process, leaving the pair trapped together inside the sheriff's office with little hope of escape. As they wait, pinned down and facing what may be their final hours together, Marshal Graham rides into town to take control of the situation. Meanwhile, Jonathan, Mabel's husband, finds a letter she left behind explaining her decision. The film charts the consequences of Mabel's choice — loyalty to her husband versus desire for Paul — against a backdrop of moral ambiguity, violence, and the constrained freedoms of the frontier West.

Sources: onceuponatimeinawestern.com, IMDb (search result snippets)