the mercs

The origin is from the noun, mercenary. A mercenary is someone who will carry out combat missions but aren't part of an official organization, like the military, or CIA and are usually not motivated by justice or an allegiance. (They are usually motivated by profit) Activities associated with these "rogue agents" or "guns for hire" usually involve violence, and often killing. So the slang verb "merc" generally means to perpetrate violence, usually killing, but can just mean to beat someone up (usually to teach them a lesson/send a message). Or it can be used figuratively, as in, "that mc just got straight merc'd in that rap battle". Similar figurative expressions could be, "Man, he just got smoked!", "Dude, you totally slaughtered that guy", "Yo, she schooled that chick", "When my boy rocks the mic, he be killin' any sucka mc that steps to him". Like many slang terms, there is an original way to spell it and variations on spelling that can become widely accepted (it is slang after all, so normal grammar rules do not necessarily apply). Merc is the origin spelling, whereas murc, murk, merk, etc are alternates. Like some people write murda instead of murder, dood instead of dude, or dat instead of that.