Yup, get to work and have a nanny at home. If you are government , thats the law.

You are not a bad person at all! I think it's very normal for plans to change once the baby arrives, so even if your boss was being reasonable, I wouldn't worry about it. (I do think it's sketchy if someone intentionally misleads an employer about their plans, but you didn't do that.) You have to make the choice that's right for you.

That said, maybe you could see about going to PT instead of quitting entirely? If you still have any interest in working there.

Also, don't burn bridges! Handle things as graciously as you can and there won't be any "black mark" on your record. There is no real "record," just a boss who could possibly give you a bad recommendation in the future.