Don’t worry, you will get several options to let him suffer for what he did to his daughter and wife. Also, I agree with you. Maria’s father deserves the slowest and most painful death possible for the things he had done.
Also, you don’t have to worry about corruption in this case. You only get corrupted if you commit severe crimes like killing or harming the innocent. You won’t get corrupted, if you let certain people, like Maria’s Father, pay for the unspeakable things they commited.
Yes, Kitsune are as well vulnerable towards certain Water based arts.