OKay....I *REALLY* don't get the impression that this man is a dangerously violent guy, who is going to become more and more violent as this baby becomes a better and better communicator.
I think the *reason* this dad is lashing out physically is *because* of the poor communication.
He's changin the diaper, the baby fusses "Don't you cry, don't cry, don't cry" the dad is thinking to himself..."Waaahhhh" the baby cries..."shhhh shhhh shhh stop it, stop it" the dad says "Stop, please stop crying now, what do you want, you just ate, what do you want??!!" - "waahhhh wahhhhh wahhhhhh" the baby says. "No!!" *bottom smack*. "Oh no, oh baby stop it, don't cry anymore" dad says to himself.
I really doubt that it's "Shut up kid, or I'll give you somethin' to cry about you damn brat!" - smack smack "That'll teach ya!" - THAT, you run screaming from...directly from your house, to a lawyers office to find out what your rights are. But the former...the top description...that is not the action/thoughts of a violent man. How many of you mama, based on what the OP described, would jump to assuming the second scenario, before the first? Can't you see the first, as fitting the original post a bit better? I mean, come on!
I'm not saying this man should be commended for his parenting...or that this mother should leave her child alone with him....but from all the OP has says, this is NOT charecteristic of her DH, he DOES feel badly about it, KNOWS it's not okay and DOESN:T WANT TO DO IT. The fact that he says it's his instinct, is probably half correct...this man probably has something in his background which causes this to be his kneejerk reaction to the babys crying.
Many many men don'tknow how to deal properly with a crying baby, especially at first...some of them try and try, but end up feeling bad at it, defeated and start developing bad habits as a result. For most men who struggle with knowing how to meet an infants needs and the frustration that comes with that, the tendency is to hand the baby off to a mama who could really use the break and a partner who could handle the baby on his own while she rests, cooks, etc...so, you know, that can be annoying, to have a spouse who sort of gives up and just can't do it. But another more shocking and dangerous reaction, is to sort of snap and lash out. Yell, throw something, or, in worst case....shake a baby or hit it.
*I* think this mama needs to make sure her DH is not alone with this baby, for the fact that his frustration leads him to lash out..and I would not want the OP to take a chance and have him lose it and shake the baby or something as horrible...but I think that she is lucky, to have caught this ugly pattern of frustration at bottom smacking. As completely shocking and gut churning as it is, to imagine anyone hitting an infant...I think it's important to try and think of where this poor dada is coming from too...he probably feels like a real piece of crud for not having the skill set to manage his baby girl when she is in distress. Seriously, some men would rather scratch their own eyes out than hear a baby cry for more than two seconds...it just sparks some crazy feeling in them, they hate it so much..
My own DH totally cumbles at the sound of prolonged baby crying...he also happens to have a lot of patience for things smaller than him, which need his help...so, that coupling led him to become completely dedicated to finding out how to soothe her...and for not having boobies...he REALLY learned some awesome tricks for getting her soothed. but it could have been different...it could have been.
All I'm saying is, when you get married, you promise to help this person you love through WHATEVER comes for you...sickness, health, all that. I'm not saying your marriage comes before your childrens safety...but IMO what the OP is dealing with, is one of those harder things that EVERY marriage will be faced with.
Stop making this dad out to be some sort of horrible human being...he's probably NOT. There is no "good and evil"....in this world there are GREAT parents, TERRIBLE parents...and in between...you can find some GREAT people...with TERRIBLE parenting skills. I would be willing to bet, that this man is one of those...and he deserves the benefit of the doubt.