This isn't about murder, but you kind of have to wonder if Mark went completely off the wagon due to that drink that Ward gave him. I know it wasn't all alcohol, but it still might've happened. So that would be another thing.
#3 I'll take Ken Franklin over Greenleaf. The opening scene with Franklin pointing an unloaded gun at Ferris, foreshadowing the terror he will later inflict; toying with the emotions of Joanna Ferris by making and receiving the phone call ("I'm sure it's nothing but a practical joke."); the 'toasting' of the dead Jim on his couch...all those things (and a few more) point to a guy who enjoys this way too much.
I'd have to go with McVay over Justin Price, despite the horrible acting of George Wendt. McVay's - as portrayed by Wendt (so maybe he does deserve some praise) - comes off as completely cold. He kills his own brother and shoots him in the back of the head to do it. Price has to cover up one death and strangle another. He's visibly shook by what he had to go through to kill Linwood. He plays it coolly (and is portrayed by a MUCH better actor than Wendt) but McVay is a cold, cold fish in the planning and execution.