If you were to identify George in any way with Lennie, it would be as parent to child. It must be this way because Lennie is much like a child, with a giant's strength. George does the very best he can with Lennie, reminding him over and over again about how to behave, when to talk and not talk, where to hide, etc., but there is a limit to what George has control over. Since George is the de facto parent of Lennie, he should never have left Lennie alone at any point. But parents cannot over-protect their children -- in time, children learn to take control of themselves and their behavior. However, Lennie is permanently stalled in childhood, albeit with an adult's body and strength. In the end, George could not control Lennie's actions and therefore could not be held accountable for them.