Elliot's descent into the state of mind that he's currently in seems to have been brought upon him by the suspected death of his father. I say suspected because it's entirely possible that Mr. Robot IS in fact Elliot's father, alive and well, and his "death" only exists in one of the memories that Elliot has created in what appears more and more to be a systematic loss of memory. Darlene even says "You don't remember me again?," which clearly means that Elliot does this maybe not regularly, but often enough that it's hard for her to handle.
Anyway, IF Mr. Robot is in fact just a manifestation of memories for Elliot, it makes total sense to me that Mr. Robot would take on the form of his father. As I mentioned, the death of his father clearly was one of those events in a young person's life that shape what they are to become. Elliot's descent into what seems to us as the viewers to be a state of madness was CAUSED by the death of his father. So, he's using the memory of his father to justify what he's doing, and his imagination has conjured up this version of his father to "help" him with this plot against Evil Corp.
That's just my take on this so far.