Maybe you should look it up. Pretty easy. Nark predates Narc and has nothing to do with narcotics, unlike Narc.
The term “copper's nark,” meaning a police informer, showed up in Britain in the late 19th century, but “nark” itself had been used earlier in that same sense to mean a nasty person. The dictionary’s earliest citation for “nark” used in the snitch sense is from The Vulgar Tongue, an 1857 slang dictionary that defines it as “a person who obtains information under seal of confidence, and afterwards breaks faith.”