This is an excellent topic to bring back to the front page as Rosh Hashana approaches.
One of the best things about Chaim Walder’s excellent books is the way he brings out the importance of treating others with kindness, as well as the long-term rippling destructive effects of bullying and cruelty.
I think that the thing that frum people regret more than anything else (when and if they take the time to think about it) is unkind, uncaring and just plain mean actions taken toward others, including those taken when we were too young to know better.