In Of Mice and Men, I think George performs a "mercy killing," not murder, on Lennie, his long-time friend.
To convict on murder you need: 1) the act of murder and 2) an intent to kill which is intentional, purposeful, malicious, premeditated.
So, while George's mercy killing is intentional, purposeful, and premeditated, it is not malicious. The two men had not had a public fight; there's no signs of scuffle. George was a type of guardian to Lennie, and his role was that of protection.
According to California State Law, George may not be guilty of murder. There are exclusions, according to Voluntary Euthanasia Under the Law:
- Unlawful killings without malice or intent are considered manslaughter.
- Justified or accidental killings are considered homicides. Depending on the circumstances, these may or may not be considered criminal offenses.
So, I tend to think it is voluntary euthanasia, or manslaughter, or justifiable homicide and not murder, because there is no malicious motive on George's part. He knew that Curley and the posse were out for blood, and so he performed an act of mercy on his friend to prevent the inevitable.