George goes on a long rant after Lennie complains about not having ketchup with his beans. George knows deep down he would be better off without Lennie, but he wouldn't abandon him. Sometimes it's better to keep quiet.
Let's do it now. Let's get that place now.
Carlson wants to put down Candy's dog because he's old, stinky and suffering. Candy doesn't want to do this, however, because of the bond that he has formed with the dog. This is why Carlson offers to do it for him. Carlson eventually gives in, and he lets Candy put the dog down. This shows that it's sometimes better to make your own decisions.
Lennie snaps Curly's wife's neck after she complains about him grabbing her hair. Basically, he repeated his mistake back in Weed, where he got in trouble with another girl because of her dress. Clearly, Lennie should keep his hands to himself.
George shoots Lennie as he hears the other guys coming. In the end, he knows it's the right thing to do, even if he didn't want to or plan to at that moment. In the end, sometimes the hardest choices require the strongest wills.