Suppose, I helped a person in his tough times. I stood by him in his financial crisis and made an all-out possible effort to help him overcome his distress. Therefore, I was surely a benefactor to him.
A few years later, he stole my undergraduate degree certificate and started blackmailing me.
What do you call a person who harms (or tries to harm) the benefactor?
In Bengali, we use the word "কৃতঘ্ন" (pronunciation: Kritoghno) for such persons. Google translate shows that the English counterpart is "Unreasonable". This is nowhere near to the idea!
So, what do you call such a person?