You shouldn't get your coworkers in trouble.
John is at work and sees his coworker Tom taking a few sheets of office paper and putting them in his bag to take home for personal use.
John wants to do correct Tom's behavior.
John informs Tom that taking office paper for personal use is against company policy, and he could get in trouble.
John tattles on Tom to their boss to get him in trouble for taking the paper.
Tom thanks John for letting him know and Tom maintains a good working relationship with Tom.
John's relationship with Tom is tense in the future and his other coworkers trust him less.