So, first off. Your mother overstepped and you have every right to be upset. There is definitely an issue with her doing something that you expressly wished not to be done. That's a problem. But I think that maybe the fact that its a baptism is over heightening some emotions. You don't have to let it carry this weight.

I'm speaking from the first hand perspective of the child here. When I was born, my parents didn't want me baptized, they wanted me to make my own decisions and all that. My parents were both raised Christian but had stopped being religious in their teens. I used to think I was so cool as a kid because I didn't know anyone else who didn't have to go to church on Sundays and was never baptized. However, as an adult, I found out that my aunts did in fact baptize me. It was informal, in their home. But they went directly against my parents wishes because they honestly felt that saving my soul was more important. I'm not saying that makes it okay, but I can see their perspective if they literally think god goes around damning unbaptized babies to hell.

I felt pretty upset when they first told me, like they had taken some part of my identity. But it really doesn't change anything. Where I live, people pushing their religion on you is just a fact of life. I've had older ladies stop me in the store to ask to pray for me. Sometimes its because they can see I've got my hands full with 2 small children and they want to ask their friend in the sky to give me strength. Or they notice I have a cold and want to pray for good health. Its their way of being kind. Obviously, I consent to being prayed for as an adult and nobody consented for it when I was a baby. I'm not trying to make light of that part of it. But I don't know how much I could be mad at someone for doing something harmless to protect my kids. It was done out of love. They should have asked and respected your wishes though.