He's wrong. You guys agreed not to use physical discipline, and he needs to stick to that. How long has your daughter been hitting people? Let me be clear, I don't think he should be hitting her regardless. But if the hitting has been going on for a while, I think both of you need to find a new way to discipline her. You can still do gentle parenting, but differently than whatever you've been doing because it isn't work.
By hitting her back, he is showing your daughter that it is ok to hit - and so is reinforcing your daughter’s behaviour. It is the same as telling your daughter off for swearing, by swearing at her - which is extremely confusing for a child. If I were you I would gather together some resources/research that teach about why hitting is not the best method of discipline, and ask him to read the materials and evidence. Maybe then he will allow his opinion to be challenged by the findings of scientists and developmental psychologists. E.g https://www.developmentalscience.com/blog/2022/2/10/hitting-children-leads-to-trauma-not-better-behavior
If he is not willing to be challenged at all on this, it is so understandable for you to think about divorce out of your desperation - you’re a good mother for wanting to protect your children and raise them in a healthy environment. I would always suggest first going to couples therapy - his parenting style is likely just learnt from his own upbringing and so never been questioned. Therapy might help him to question why he believes what he believes and to see that it likely had an adverse effect on him when he was a child and will also do the same to his children
Hitting her where? Same as she has hit someone and not a spanking?
I don’t think you’re overreacting. I told my husband that I would leave him if he was ever physical with our children. The only issue is, if you do leave him, he will probably continue to do the things that you don’t like, just without your knowledge. I would really try and speak to him and explain how much this means to you.
I don't think you're overreacting particularly, as you're absolutely right that hitting back is not the way to teach that hitting is wrong, it's just illogical. He's not going to be hitting when he's calm and rational, he's going to be hitting when he is angry so he's showing her that the correct way to react when she's feeling angry is to hit. I'd also be worried that he doesn't seem to be listening to you, and is dismissing something that is so important to you and a fundamental value you hold. I'd be trying to explain to him again why this is so important to you. You could also find something for him to watch and say "I know you're unsure about this, I wanted to share this to explain more my point of view and see what you think". I'd also be clearly saying to him "I will stop you if you hit our child, my job is to keep her safe". Maybe that would make him think about what he's doing, would he actually be willing to hit you to then hit her.