Should I feel bad for abandoning my mother?

First off, you have not abandoned your mother. Sounds like you're doing an awful lot for her. You are probably correct though that you should have put mom in a nursing facility, as after reading your profile, it certainly seems like your brother is barely able to care for himself, let alone your mom. It's not too late to look for a nice facility to place her in. She can still remain under hospice care while in the facility too, so she will have extra sets of eyes on her, and they will still send their aides to bathe her there, and the hospice nurse will just visit her there instead of the home she's in now. In the meantime I would like to think if the hospice nurse or aides that are now coming to the house, see any neglect or abuse that they would report it. I'm sorry that you're under so much stress. Being a caregiver is extremely stressful, and add to that life's other stresses, and sometimes it can just be too much. You are doing the best you can, so cut yourself some slack. However don't tolerate any kind of abuse from your brother. That is unexceptable. And yes, you are ok. You're just human like the rest of us.

Your Mother wants to stay in the home with her Son caring for her and I honestly think that is kind of the beginning and the end of it where that is concerned for now.
Your brother sounds as though he is stressed to the max (unless this has been his lifelong norm). He is diving under the covers, and he is wanting to go out to the casinos? Sounds like he truly needs a break. I think we can't know if you are talking gambling addiction or just loving the trip to the casino for a break (I adore that occasionally and miss it).
Have you had a normally great relationship with your bro that is now deteriorating, or has it always been a little adversarial.
Can't say how much I admire your trying to help; and I would hope you will continue to do that to the best of your ability.
And hey, I think you are better than OK. You are trying to help, and are frustrated. We all have our problems and we all have our limitations. Try to be as gentle and loving with all as you are able, and as understanding. That's about the best you can really do.