He didn't want to rat on his partners, but ended up telling their secrets to the police.

rat 在这里是动词，本意是"老鼠"，作为俚语使用时表示"告密、出卖"，指像老鼠一样偷偷摸摸地背叛他人。rat on someone 这个短语特指向当局举报或出卖同伴的行为。