Just wanted to add that I think the cancer analogy is a good one. The particular underlying mental illness he has, will have a mortality rate. Just as different cancers have different mortality rates.
So, it is reasonable to expect that 10% or 20% or whatever it is, of people with a particular illness, will die from that illness, sooner or later.
That isn't to say that any individual death is inevitable, that everyone shouldn't be working to reduce those percentages, or that he shouldn't seek and accept all the treatment he can. It is to say that sometimes, all the treatment, love and support in the world will not be enough and death, from a serious disease, with a known high mortality rate, will occur.
I think for your own sake, that viewing him as someone with a serious, potentially mortal illness may be helpful. It helps to remove the day-to-day sense of personal responsibility for the jeopardy he finds himself in. You can help, by offering continuing steady support and encouraging him to seek and accept professional help consistently. You cannot intervene in such as way as to prevent death, when his health has declined to an extent that he is in crisis and death is the likely outcome, any more than you could with cancer, or with the failure of another organ. You cannot save him.
To be in a position to offer steady support, you need to look after yourself. Tell him clearly that you don't want to hear about how he would do it, or anything similarly upsetting and destabilising. It sounds as though either, you are setting yourself up as a pseudo-psychotherapist and asking probing questions, the answers to which you cannot deal with. Or, his behaviour towards you is somewhat vengeful - of the 'if I'm going down I'm taking you with me' variety - which will be because you are the person who is there, who represents 'normal life' and its unattainability, to him. Nothing personal.
So, look after yourself. Seek help for yourself. Focus on other things too. Make it clear to him that this is the only way you can help him. encourage him to accept all the professional help available.