Of mice and men? nice book
George killed Lennie because Lennie accidentally killed Curley's wife by accidentally suffocating her. George knew that Lennie would be hunted down by Curley and his mob, thus he felt like he couldn't protect Lennie anymore. George believed that killing Lennie would help him suffer less then face the slow torture Curley would have inflicted on Lennie. I do believe this was justified.
Goodluck out there, partner :-)