The question is how much to share of it with your readers?
While your John Doe might have had a lousy childhood with a mother who beat him, an older brother who bullied him and an uncle who dressed as a clown on his off days, how much of that should you throw into your audience’s laps?
You don’t want your backstory to become the story.
In John Doe’s case, I’d definitely reserve some time for his mother issues, but the big brother stuff I’d keep to myself. As for Uncle Bozo, I’d throw him in as a personal aside only. For example: “John supposed he should be more concerned about his eighty-year old neighbor peeking in his windows, but he’d had an uncle with a clown shoe fetish so Mrs. Fitz’s one irregularity hardly made him flinch.”
Bottom line: Dole out the backstory wisely. All dirty laundry should not be aired.