I think that’s exactly the point of mindfulness? To not necessarily focus on thoughts but allow the mind to become uncluttered by thoughts. To follow the breath and when the mind wanders to always bring it back to the breath. Being mindful is about being aware of our thoughts, yes and some of those thoughts may go to dark places, but that’s exactly the point. It’s not about analysing or even identifying with those thoughts (be that positive or self destructive), for me it seems to be more about returning to the blank slate: that we were all born as innocent beautiful beings with incredible potential.
And then again, perhaps mindfulness is different for everyone, a deeply personal experience. But it should by no means replace therapy when needed. Those individuals who take violent paths, such as murders etc, need to work through deep therapy. For someone so traumatised, simply meditating isn’t going to change their core beliefs. So I don’t think the problem is mindfulness but how we approach it and perceive its value.