Context

In Chapter 3 of Of Mice and Men, Carlson comes in and complains again about the smell wafting off of Candy's dog; he kindly offers to shoot the creature with his trusty Luger. 

Candy loves his old companion and can't bear the thought of killing him, but Slim, surprisingly, sides with Carlson: he'd want to be shot if he were old and crippled. Since Slim has handed down his verdict, the dog is as good as dead. 

Carlson shoots the dog, and a while later, Candy tells George: "I ought to of shot that dog myself, George. I shouldn't ought to of let no stranger shoot my dog."

If you've read the ending of Of Mice and Men, you know what a bundle of foreshadowing that is.

Where you've heard it

In the (paraphrased) words of George R. R. Martin, you'd better not condemn someone to death unless you're willing to carry out the execution.

Pretentious Factor

If you were to drop this quote at a dinner party, would you get an in-unison "awww" or would everyone roll their eyes and never invite you back? Here it is, on a scale of 1-10.

Owning up to something ≠ pretentious.