Bite the Hand that Feeds You
Morder la mano que te da de comer
To attack someone who has helped you, to turn against someone who had done good things to you.
This saying, since the 1700, has referred to an ungrateful dog which bit the hand of its owner in the very moment when it was being fed by him. This expression is now used to refer to those who act against you after you have spent time helping them (not literally feeding them).
-Oh, Gosh. I am so freaking mad.
-What happened to you Alfred?