I avoided reading this since it was posted because I happen to find Rita Skeeter one of the most appalling characters in the series... but the lure of Karkaroff was too much. And I must admit that I'm very impressed with the way you've characterised him. He's so... normal and yet not. The little zealot he becomes after sex, all high on that and his political discussions.
Fascinating, as well, that Rita saw fit to dob him in for the benefit of removing him as a threat to the 'greater good', even if she did get something out of it herself.