I don't know if self-destruction is part of the group's mandate per se, but I do think it's a more or less inevitable result of it. Being a Torchwood member is, as Suzie pointed out in the premiere, a dream job, but it is an adolescent's wet dream: no rules, no oversight, being free to do whatever one deems necessary by virtue of the simple fact that one is better than everybody else. It's a type of power which is inexorably corrupting, I think, and the show has done a fairly good job of showing that so far.