to bite the hand that feeds one
Idiomatic translations of "to bite the hand that ..."
Meanings of "to bite the hand that ..."
It's used when you do a good deed for someone who´s ungrateful and mean. He/she will never make it up to you, let alone thank you!
To turn against/offend/harm somebody that is supporting/helping/benefacted one
Tratar mal alguém que já te ajudou anteriormente.