If an animal preys on another animal, it catches and eats it.

The spider preys on small flies and other insects. 蜘蛛捕食小苍蝇和其他昆虫。

to hurt or deceive a group of people, especially people who are weak or can easily be hurt or deceived

He would attack at night, preying on lone women in their twenties or thirties. 他总在夜间出没，袭击二三十岁的单身女子。

It's particularly contemptible that these sort of people prey on the elderly. 这种人坑害老年人真是太卑鄙了。