Tony Hale. He's typecasted as crazy all the time.
But more seriously, most of the lesser know names seem to fit better. It seems like some more well known names don't fit as well imo. Maybe it's because I've seen them as non-Spector characters, or maybe because they are well known for their attractiveness rather than their diverse acting skills.
Currently looking for a lesser known but good actor who can play a dark character who is not fully stable...
Jim Caviezel? He's very dark in manner in most films I have seen him in, and his monotone acting voice could work well for Marc.
Sharlto Copley would be an interesting choice. He can do a darker character well as shown in Elysium. He's usually seen as a more lighthearted actor, which is why his performance was so surprising.