The "Hang Nelson Mandela" material was created by a group, which Cameron was not involved with, within the Federation of Conservative Students.

Disclosure: I'm not a Conservative supporter. I dislike what Cameron is currently doing and loathe Thatcher.

[1] http://www.independent.co.uk/news/uk/politics/cameron-we-wer...

[2] http://www.newstatesman.com/media-mole/2013/12/twitter-fact-...