In the interest(s) of sth: in order to help or achieve sth 为了,有助于
In the interests of safety, smoking is forbidden.
为了安全,禁止吸烟.

Pay sth back/return sth with interest: react to the harm sb has done to you by doing sth even worse to them 加倍回击(或惩罚)
Peter pushed his sister, so she paid him back with interest by kicking him hard. 彼得推了妹妹一下,妹妹反过来狠狠踢了他一脚.