In short, since you asked several...
1. George is small, square-faced, smarter, and a father-figure. Chapter 1 would give you the best quotes on the 2nd-5th pages of the chapter. Lennie is big, rounder-faced, dumb, childish and mentally slow. He is compared to a bear in the beginning.
2. The setting given in the beginning is on purpose because it is the exact spot that the story ends. The setting in chapter 1 is by a river, more specifically a pool, just off the road on a quiet and serene day.
3. George had to kill Lennie because he would have hurt others again, and had George let someone else kill Lennie, Lennie would have suffered.
Believe it or not, Steinbeck sees the ultimate theme in this novel as hopeful. As depressing as the ending might be for us, George sacrifices everything--his dream, his friendship, his happiness, and maybe more--in order to keep Lennie from suffering. Everything in this novel is bleak, dark, and bitter except for this perfect act of selflessness. How powerful does Steinbeck think friendship can be if it is the only bright spot in this sad, lonely world depicted in the novel?
I believe that George also realized that Lennie's uncontrollable strength would continue to get the two of them in trouble if they somehow managed to elude capture following the death of Curly's Wife. There is little chance that Lennie could have escaped the people sent to hunt him down, so George acted out of friendship by putting him out of misery himself. Although Lennie never meant to kill Curly's Wife--or the puppy or the rabbit or the mice--his loving attempts to caress always seemed to turn into tragedy. For his own sake, George could not allow himself to be involved in another such episode.
The reason that George kills Lennie is because he does not want anything worse to happen to him. I know that sounds weird, but George thinks that it would be really hard on Lennie to go to jail, for example. In addition, I think he realizes that Lennie is going to get killed and he figures it would be better if he did it than having someone else -- that way Lennie could feel fine right up until the moment he died.
The reason why George had to kill Lennie in the first place is that Lennie killed Curley's wife. He is surely going to be in big trouble, even if it was accidental.
The reason that George had to kill Lennie is because earlier in the chapter (6, if you're referencing) Lennie killed Curley's wife in the barn, and Curley found out, and went out to kill Lennie. George has the common sense enough to know that if Curley were to get to Lennie first, then he would either kill him in a very painful way, or have him locked up and miserable. Before killing him, George tells Lennie to sit down and look out over the pond. George describes their dream of owning a farm so vividly that Lennie almost starts to believe that they are living it. Lennie is very happy and excited when George finally shoots him. Lennie doesn't see it coming, but it was very fast and pain-free so he didn't even have time to realize it. Slim comes up afterward, comforting George and saying "You hadda George, I swear you hadda."
This does not really answer the question but it is just an interesting metaphor to know.
We all know that Lennie cares for things for flurry things such as puppies ans mice but unfortunately kills them, i believe that is an extended of how he cares about the dream but ironically kill it.