Nicely done. The transition from Barty's innocent, childlike view of the world in the beginning to his corrupted, twisted thought processes at the end is fantastic. Great depiction of obsession and the subtle dangers thereof.
One of the more intriguing aspects of this is Rabastan's motives throughout. Surely he would have seen the obvious potential value in turning the son of an important Ministry official to Voldemort's side. Could he have ever seen Barty as anything more than a conquest and a plaything?
Yet Barty, caught up in his obsession, never thinks to question this. Even in the end his primary concern is saving Rabastan.
The last few lines are just so dark and chilling, because we all know how things went for Barty from that point forward. Excellent.