George killing Lennie is very immoral because Lennie does not know any better. Lennie is not killing people and animals on purpose he just underestimates his strength and does not realize that he is doing wrong.
A biker's helmet is often called a brain bucket. Not wearing one is literally, "a no brainer." The pun also implies that one riding a bike without a helmet has no brains.
Since historical fiction usually talks about a certain era in history, I'd say the correct answer is that 'it's set against a historical backdrop'. hope it helps you
