There are many kinds of pain and no one will feel the pain of another. There are many who are divine and no one knows peace or the divine.
You talk about karma and how just it is. How could you, in your state, judge the innocence or guilt of the small being who sinned not? Yet you find him in guilt of some forgotten life and declare him guilty.
Who is really guilty or pure? I believe the one who judges the small being is in fact the sinner.
The priests who judge must first know all of lives passed or know to seek peace and learn to love, free of pain – now or tomorrow.
Let Creator be the judge.