I need some help not really understanding...i just need some ideas
The issue you bring up isn't simple, though. Utilitarians believe that a moral act shoud be judged on the basis of what brings the greatest happiness to the greatest numbers. Thus, justice for a murderer would be essential, but not necessarily taking justice into one's own hands.
And don't make the mistake of thinking George kills Lennie for the sake of Slim or anyone else. He kills Lennie for Lennie's sake. He doesn't want Lennie to suffer for something he doesn't understand. He doesn't want Lennie to suffer at all, just like Candy's dog is shot so it doesn't suffer anymore.
George takes mercy on Lennie. The happiness of others has nothing to do with it.
I think I had it backwards. I thought the utilitarian was all about what makes us happy. The goal being to pursue those things that make us happy or are good for us. I thought universalist was the greater good for the greater number of people. OopsJ
Utilitarians believe that a moral act produces the greatest good to the greatest number of people. I would say George is a hero because he kills Lennie and spares many people the pain of seeing Lennie put on trial, etc. George wouldn't like it. Lennie wouldn't. Neither would Slim. I think perhaps the only person who would think it was good was Curley, who would want revenge.