I don't know about can't be mad. I think it was the manner in which it happened not why it happened. The OP did say she realized it was her fault they got into the yard. The 'kid' is 18. Technically he is an adult and needs to learn what that means. I don't care what happens I expect my neighbors to be civil. If they are not, then I have the option of not being civil also. The OP is taking responsibility for what her chicks did, she made it right by fixing the issue with them going into the yard, she is working on rehoming. Yes she can tell the neighbors that. Will it help. Probably not. I tend to assume that kids act like their parents. That whole they learn what they have seen issue. However the neighbors kid has to take responsibility for how he acts also. We each have to take responsibility for our own actions. The OP just needed to rant a bit about the kids attitude before she dealt with the situation. Its better to pull in outside voices and get calm before you try to work out an issue.
I would talk to the parents. I would let them know what happened with the chicks, what you are doing to fix it. I would also let them know how their children acted and the language they used. They might be interested that if their son had actually stomped on a chick he could even face animal cruelty charges. There are proper ways to deal with conflicts. Cursing, yelling and violence are generally not the accepted ways to deal with conflict. Their son is adult so this is stuff that can go on his record. I am sure he never would even think of that. Just do whats right and then hope for the best and expect the worst.