The best way to answer this question is to work backwards. At the end of the book, George had to take a gun and shoot his best friend in the head. George knew that the other men would not be kind. They would lynch him for an accident. This shows that Lennie does not belong in society. This society does not know what to do with someone like Lennie.
Before this awful event, Lennie did not fit in on the ranch. The men tolerated him, because of his strength and because George vouched for him. One man, Curley, started a fight with him and would have beat him (perhaps even to death), if he had the chance.
Before life on the ranch, Lennie and George had to run away, because the men they worked for thought Lennie attempted to rape a girl. This was not even close to the truth, but they did not take the time to find out what really happened.
All of this shows that the world in which George and Lennie lived was cruel. People only look out for their own interests, and there is very little compassion. It is a world that does not take the time to get to know people.