Besides being the only past DADA Professor that would be capable of stealing something from within Hogwarts, Snape is the only one whose identity is still in question and has ties to Voldemort.
2
如你所知,大家都很乐意去拜读下霍格沃茨黑魔法防御术教授的招聘广告。
You know, we'd really like to read the job adverts that Hogwarts publishes for the Defence Against The Dark Arts teaching post.
Trivia: Professor McGonagall was appointed Headmistress of Hogwarts after Dumbledore's death, but was ousted by Snape when Voldemort took over the Ministry of Magic.