You already know his character. How he behaves. Everyone agreed his behaviour was very wrong.
You cannot placate a person like this. They don't behave normally, so they won't react how you want them to just because you ask them...Or they wouldn't be like that in the first place.
From what you've said, He's never apologised for his behaviour. He repeats it.
If you ask someone who behaves like him just to the Christening. He will resent it. He will display his resentment. How do you think he is likely to display this as he watches everyone else making their way to the party?
Is he so religious that he will appreciate the solemnity of a Christening? Is he so well disposed towards you OP, the person he's happy to verbally abuse, that he will enjoy the event and be happy for you?
He is not someone who likes or respects you or even wishes you and yours well. WHY would you want him there?
To keep others in the family happy? How will his unpleasant presence achieve that. It will just make you resent the others you've kept happy.
Why is their "happiness" more important than your own? I think their "happiness" is more relief that they don't have to be in the firing line from BIL, that they don't have to be the ones standing up to him. That while he's picking on you, he's not picking on them!!!
You only get one chance to christen your newborn. Its a joyful time, and the people attending should be those who love you and your new baby and want to welcome them into the family. I don't think your BIL fits that description. Don't give him a chance to ruin a key moment in your lives. And you should have no guilt whatsoever about this.
Keep saying NO and take the flak that goes with it.
The push back you get will be from people who don't have the guts to push back with BIL.
Any flak you get will be mild compared to your own upset if he wrecks everything and the stress you will be undergoing in the run up to it, imagining what he could do.
If you want to offer an olive branch say that you will happily be civil to him at the next family gathering ( not at your house) and leave it at that.