The most alarming part of that conversation for me was "A few weeks ago I punched a housemate in the face ten times, breaking her nose;"

If I was having the conversation, I would ask him more about this, and talk at least a little bit about how he could stop hurting other people.

I'm not sure what you mean by the bottom of the problem. I will say some things that I think are problems. These are speculations. I don't have enough information to be confident in these answers.

1) I think it is a problem that, as far as I could tell, no one intervened and taught him not to be abusive after the punching incident. This is a problem with society.

2) I think it is likely that he has a brain injury from a head injury of some sort and/or from taking drugs such as meth. He mentioned both a head injury and meth. I would say to get treatment for brain injury, but doctors are still pretty clueless about how to treat brain injuries, though there are experimental possibilities.

3) I think it's possible that he's a sociopath, but there's not enough info to figure that out. The combination of not finding pleasure in life, and feeling no remorse, and not thinking about the effects of his actions on other people is suggestive. (Although there can be other reasons for that.)

4) I think there's likely something else wrong with his health. Maybe bipolar, since the bipolar meds are helping a bit, but I'm not convinced that accounts for everything.

5) I would guess that he did not have opportunities in childhood to be self-directed. His sense of not knowing what he enjoys, or what he wants, or how to make a plan, might be from lack of education and training in those areas, not just from physiological problems affecting his judgement.