Based on my knowledge of the comics as well as what they discussed on Talking Dead, I can explain Abraham's flashbacks for people who don't quite get what's going on.

I'll spoiler tag it in case you want to see if they explain it further in a future episode, but I don't think they will.

Explanation of Abraham's back storyAbraham and his family were surviving the apocalypse with his friends/neighbors. As the story goes his family was raped while he was out on a supply run. The guy you see him beating the trout out of with a can of food is a guy he thought he could trust. Abraham beat the guy to a pulp and killed him viciously, and his family was watching. They were very terrified as they were seeing this relentless brutal soldier/monster in action and were afraid of him. They left and by the time he found them, it was too late.