I think the flashback was a couple episodes too late - they were so intent on showing him as this...grounder-hating dude who just wasn't interested in anything other than grounder-revenge, then damage was done, then they showed us a bit of his past. I don't think it should work that way. Plus after that, he wasn't redeemed in any sort of way so what was the point
(At least, this is how I remember it - I haven't done a full season 3 rewatch yet)