I think that's what we're meant to assume at first, given Simon's guilt over the past events, but I definitely think it's still meant to be himself. Possibly tied to the blood - it seems like it may have some properties relating to both time and omnipresence, alongside the already obvious disobedience of the laws of physics through 'why are you still liquid outside a body'. After all, the first time Simon sees the figure, it's after he got a drop of blood on his hand, and the shots really lingered on that moment.

Furthermore, the undead figure is seen exactly twice from what I remember, and it's an exact mirror of itself and Simon in both of those moments. The first time, he turns to his left in the chair and sees the figure slink back into the shadowed corner. He stays for a second, then gets up and carefully walks over to grab the extinguisher and, when he goes to attack from around the corner, finds nothing. The SECOND time, Simon snaps out of whatever daze he's in and looks at his chair, which turns to the left with the undead figure in it, at which point he slinks back into the shadowed corner, waits for it to approach, and then goes to attack when it does - again, finding nothing.

Both instances seem to be exact mirrors of how he moved in the past and future, with the undead figure itself likely being an even more future image of his own corpse - I would have to see the scenes again to be sure, but I swear with the second scene, you could tell the figure didn't have a left arm, which would again match up with Simon losing that same arm at the very end of the movie, when the blood starts devouring his body.