I guess I will be the outlier here and go against the tide.
I won't tell you that you are the bad guy (not that anyone here has said that) and I won't tell you to stay. You have the right to take care of yourself and the responsibility to take care of your child. I know this issue from both sides as both my husband and I have dealt with depression. Fortunately for us, neither one of us has been hit with it at the same time and the other is able to pick up the slack. We both respond to treatment very well and have a great life together despite the challenges.
I am wondering if your mother-in-law can be of any support for you. She sensed that something wasn't right. That isn't a criticism of you but rather a parent being concerned for her child. Perhaps together you can provide a support system for your child.
Your wife has to be able to take charge of her recovery or there won't be one. You can't make her do that. Taking care of yourself and your child does not mean giving up on her but rather allowing the responsibility to be her's for what is going on in her life. I believe in loyalty and committment but I also know that you are the only one that can make the decisions for you. You have received some great advice here about support and places to find information.