Of course their punishment in such a case WILL come about regardless of your sentiments. They did a terrible thing to a Yid without remorse and they will be called to account. That is between them and Hashem. It’s not your cheshbon or dependent on your will.
Your kavanah, as I understand it, should be something like: Hashem you will do what is best for this persons Neshamah and to fulfill Your Justice. But please whatever happens to this person don’t let it be for my sake. Let it be for Your sake. Do not punish them because I am rightfully angry. Punish them because You are angry. I am moichel them as much as the ratio of my humanness to my Malachness will allow. That is my part, that I do because You desire it from a Yid, for whatever Your reasons. And You will surely carry out Your Justice.
That’s basically how I understand it. If the theologic scholars enter this thread, I can’t quote the sources.
This is a personal note from me to you.