He ended up ratting out his partners to the police to save himself.

ratting 是动词 rat 的现在分词形式。rat 原指"老鼠"，作为动词时引申为"告密、出卖"，因为老鼠常被视为背叛的象征。rat out 是固定搭配，表示"出卖、告发（某人）"。 rat out his partners 指"告发他的同伙"，这里的 ratting out 强调背叛行为。