养虎伤身

yǎnghǔ-shāngshēn

[bringing up a tiger one sustains injury himself--to be injured by one whom one has been kind to] 饲养老虎, 伤及自身。 比喻纵容坏人, 害了自己

兀术听了, 大叫道: "罢了, 罢了!此乃养虎伤身也!"--清·钱彩《说岳全传》