You were a kid. Kids makes mistakes. You've learnt from it though and I expect you are and will be a better person because of it. They seem to be doing fine now so if anything this has been a positive. You did something nasty, but it's not like at least one of them wasn't being nasty too- most kids are at some point. You've felt the guilt now and you know that it hurts to be mean. You can use this to make yourself better and maybe even as motivation to be more kind.
Doing a bad thing does not make you a bad person. Don't label yourself for a mistake you made as a kid.
Hi I am from china,congratulations for your brave to tell what you fell anyway whatever good or bad.it is ok to face yourself,no one can avoid do something wrong in youth.Talk it to your friend is ok,and the other way is to find a secret place talking to yourself,speak out all you felling about it to yourself,your can argue with it just pour all you feeling out,may be this will help.
I think I found a picture of you Jokes aside. Get over it, it was in Y7 and move on. Simply learn from your mistakes and the mistakes of others and never be a snake again