Creator Worship: Mark (IRL) is already loved by his entire fanbase, but his dedication into making something of this quality just for his fans only increased this. Many have compared Who Killed Markiplier? to a YouTube Red movie that you don't have to pay for.
Epileptic Trees: A large part of Markiplier's audience agrees that the District Attorney was reincarnated into and/or is the same person as the Player Character of A Date With Markiplier.
Fanon: Just like how Celine was Red and Damien was Blue, it's often depicted that if William got sent to the Upside-Down, he'd have a pink Aura. Conversely, it's depicted that if Damien went insane, he'd end up getting Blue hair just like how William got a pink mustache.
He Really Can Act: Mark already has proven himself as an actor in the Cyndago shorts and as Darkiplier in A Date with Markiplier, but this is where people began to see Mark's rather impressive acting talents, especially when William snaps and calls out for the now dead Damien and Celine.
It Was His Sled: William goes insane and becomes Wilford Warfstache, and Damien, Celine and the D/A merge with the Evil Entity and become Darkiplier. For that matter, the fact that the series serves as an Origin Story for Wilford Warfstache and Darkiplier.
Jerkass Woobie: Markiplier. Sure, he may have devised an elaborate revenge plan on those who wronged him, but before he was corrupted by the Manor, he was happy, according to Benjamin. He and Celine were Happily Married, he was great friends with Benjamin and treated him as an equal. It's why Benjamin holds his death in such a tragic light. In fact, the reason he devised his plan in the first place is because Markiplier Manor took advantage of him while he was dealing with his depression after his wife, Celine, ran off with the Colonel, one of his best friends from childhood.
Paranoia Fuel: The Manor could be manipulating you and you'd have no idea when it'd be happening.
Unpopular Popular Character: The only people the Colonel seems to be amicable with are Damien, Celine, and the D/A. Everyone else seems to dislike him. Benjamin in his interview livestream states that he was troublesome, outlandish and self-centered and doesn't hold him in high regard. However, fans love him for his eccentricity and Cloudcuckoolander-ness.
What an Idiot!: Once Celine enters the picture and starts informing everyone that something occult is going on, it would make sense for them to get the hell out of dodge. George, Benjamin and Chef leave, but William, Abe, and the District Attourney don't, and they end up suffering for it
The Woobie: Damien and William. Damien got his body taken over by Mark and eventually turned into Darkiplier, and William lost Damien, Celine, and his own sanity in the span of one night. While Mark had his reasons to go after William, Damien on the other hand never did anything to Mark. He just happened to be in the way.