口蜜腹剑，笑里藏刀

honey-mouthed but dagger-hearted; with honey on one’s lips and murder in one’s heart;hypocritical and sinister(or malignant)
这个人～，十分阴险。He is a sinister and ruthless man with honey on his lips and murder in his heart.or:He is an evil man who has a mouth that praises and a hand that kills.