There's only four things that can happen when you throw a pass and three of them are horrible. Unless you're a shrink, you stand a great chance of getting it wrong. Unless you're s genius, you're going to bore your reader. And unless your uncle runs Random House, you're not going to sell it. Don't use flashbacks if you can help it.
Flashbacks can certainly work, but more often they simply stop the story.
Don't forget that your character has his own point of view and that how he reacts to stimulus does more to define his character than anything you could tell us by way of explanation. Enough of the kindergarten stuff. Let's move on. People believe what they already know or what they want to know. People rarely change their mind about a person just because the evidence says their opinion is wrong.