I am sorry you are going through this. It sounds like what many of us have been through, the total inability to get treatment for a very ill and loved family member. This is not your fault or your husband’s. This is the current lack of treatment for mental illness in the US today.
I also vote for you to leave and here is my logic.
You already know you have done everything you can to try to get treatment for a very ill husband.
Let’s say your husband doesn’t become violent, then you really aren’t helping by staying while he is so out of control. It’s called “enabling.” Tell him you are leaving because of his drug use. This is a normal reason for a spouse to leave. A chaotic environment is not a place for children and he will understand this at some point.
Let’s say he does become violent. You don’t control that at all. When I was four years old, my mother with bipolar one became more and more ill. She beat me up quite badly and had never hurt me before.
You can’t treat his mental illness any more than you could perform heart surgery on him (if you are not a surgeon). It’s not in your hands. Your life and your children’s lives are in your hands. His illness and drug use are not in your control.
So, yes, leaving him is difficult. But it’s the last resort left to you besides living this way.