preyed on

to hunt and kill another animal for food 捕食；猎获

to harm sb who is weaker than you, or make use of them in a dishonest way to get what you want 欺凌，坑骗，敲诈（弱者）
Bogus social workers have been preying on old people living alone.
冒牌社会福利工作者不断坑害独居老人。