In John Steinbeck's novella, Of Mice and Men, George is faced with the decision of killing Lennie or allowing Lennie to face the consequences of accidentally killing Curley's wife.
George's internal conflict with the decision about what to do about Lennie is overwhelming. Essentially, he has two choices: kill Lennie or allow him to live. If he kills Lennie, George insures that Lennie does not face the torture that Curley and the other ranchers may force upon him. Lennie's death also insures that he does not face a trial, if he makes it alive to one, where he may be committed to a mental institute (given his diminished mental capabilities).
George decides that it would be best to take Lennie's life. He cannot face the chance of Curley and the other ranchers hurting him or the chance of Lennie being locked up. George made Lennie's aunt a promise that he would take care of Lennie. In the end, this is exactly what George does when he decides to kill him.