@Paperbear I am so unbelievably sorry that you have to face this, but I am sure you can do it. Only I think having the chemo is enough - I think adding more stress to it by going through separation/divorce as well is too much.
Make no mistake, your DH is not nice, not good, and yes, eventually, you will be better off free of him. And I know that right now he is also going to bring you down with his comments and meanness. But he is also going to be of practical assistance while you go through the chemo, even if he's only sitting with the kids.
I would be worried about where you go if you split up. You have enough to focus on in fighting the illness. If you split up, you will also have to find a home and move, or get him to move out, you will have to sort out so much, with money etc too, you will have way less money right when you need nothing to worry about and won't be able to work. You will need a bedroom, somewhere to lie down and rest and get better. You will need more than ever your home.
I had a friend who went through many years of cancer treatment, and she also had a bad time with her DH, and I remember saying well why don't you just leave him then. And she said because I need him right now. I need him. To do things that I can't be doing because of focusing on myself. And when she was better, she then got divorced and is now very happy and has a new DH who is lovely to her.
Unfortunately cancer needs to be dealt with quickly, and you don't have time to get divorced first, or get a better DH to support you.
Right now I feel he's the least important thing. I don't know if you have family close to support you too, but with him, I would just try to ignore him (I know, easier said that done). Only because I worry about the alternative and how you will fix things up.
I hope you have someone to talk to. There is lots of support and you could even call Refuge or the National Domestic Abuse helpline to talk about how he's being and they may have some way of helping. xxx