According to TV Tropes, the Big Bad is the major villain who is directly responsible for the plot. Which would be our Sharp-Dressed Man, Darkiplier.
However, the Greater Scope Villain is different. The GSV does not engage the hero directly and is indirectly responsible for the events of the story… which might include the creation of the Big Bad.
The threat of the Big Bad is immediate, whereas the GSV is always in the background and out of focus. Two good examples of this are the First Evil in Buffy the Vampire Slayer, and the Senior Partners of Wolfram & Hart from the Buffy spinoff Angel.
Dark may be Mark’s “shadow,” but WKM!Mark appears to be more of a mirror version of our Mark than Dark is. And the two times we’ve seen him (in the “Meta” ending of A Date with Markiplier in February, and now in Who Killed Markiplier?) it was only very briefly.
It may very well be that WKM!Mark is the true evil, for the damage he’s caused, and the fact that he apparently has never really cared about anything other than himself. The fact that he’s willing to go to any lengths to hurt others, for any slight to his dignity… and the machinations of his plot turning a good man into a vengeful monster.
He may have only began as an egotistical actor but, by the end, what WKM!Mark did was truly evil.
We know our Mark is being targeted by Dark because of the unfortunate resemblance, and that Dark will only have a shot of finding inner peace when WKM!Mark pays for what he did. So, while Dark may be our Big Bad… WKM!Mark may be our mostly-offscreen Bigger Bad. Which is the Greater Scope Villain, in a nutshell.
Maybe this will eventually result in an Enemy Mine scenario, with our Mark and Dark teaming up against WKM!Mark to finish what was started in the Manor.
Just something I was ruminating on, and wanted to throw out there.