1. Lennie was dangerously undisciplined and lacked self-control, putting others at risk of harm.
2. After accidentally killing Curley's wife, Lennie would likely face execution for her murder.
3. George knew they would no longer be able to live and work together on the farm due to Lennie's actions.
4. By killing Lennie himself, George spared Lennie a painful death and showed him mercy and friendship until the end.