Snitch (n.+v.)

Snitch (vt.+vi.)
1 [intransitive] to tell someone in authority about something that another person has done wrong, because you want to cause trouble for that person
snitch on
Somebody snitched on me.
2 [transitive] to quickly steal something unimportant or of little value [= nick]

Snitch (n.) [countable]
someone who is not liked because they tell people in authority when other people do things that are wrong or against the rules [= sneak]:
He didn't want to be a snitch, and besides, Kevin was his friend.