I only just clued on. It was when he asked Mohinder to commit genocide but they didn't give us an actual background reason why Nathan would think Mohinder was this devoted to him (after all, Mo had been arguing against it all ep). I just thought 'that sounds like Sylar... OH.'

It's a good thing they didn't actually cast Adrian Pasdar as Sylar originally, because then I'd have to be very very sad when he gets killed.
OMG, I would have been ALL about Sylar then. Sylar's a fun villain anyway, but cast Adrian in that role and he'd be giving Lex a run for his money as my favourite villain.