odester, I want to tell you about the tenderness of fathers and newborns because I want you to have a sense of what is normal/usual.
I worked for a number of years in maternal-child health, in the mother-child unit of a hospital. Those early days as a family in the vast majority of cases are about as tender, about as sweet as they get. Men are brought to tears when someone comes to poke and prode their sweet newborn babies...when the babies cry, the daddies want nothing but to rush to pick their children up and comfort them. If ever there was a time when first-time mothers fall in love with the fathers of their children in a whole new sense of the word, it is in these early days of having a newborn together. There is more than a joy, but truly a reverence for this new little life that they have helped create. Reverence inherently precludes the possibility of violence.
*This* is instinct. Instinct generally encompasses those things that help propel a species toward survival.
Of course, there are plenty of us who have trouble with babies crying...who find that for some reason, we have a low tolerance for it. We need to take breaks, to ask for help from our partners, to plan survival strategies for those late nights.
But those tiny little cries early in the morning/late at night when a child has first been born? It is not usual or typical to have an irresistable urge to snap our fingers in our babies faces as soon as the cries start. And it certainly is highly unusual not to be able to control that impulse (notice I say impulse, not instinct).
I say all this because I think it would be easy, especially with the responses regarding leaving your husband, to think "perhaps it sounded worse than it is," or to otherwise need to return to a sense of normal and brush off the real concerns expressed here.
I really understand why some here are cautious about advising you to leave your husband for this alone. Indeed, even with highly abusive parents whose children are in foster care, I know as a foster parent that the plan is never supervised visits *forever.* As the child grows to an age when the parent behavior is less risky (for example, as shaken baby is less of a risk), and/or as the parent demonstrates that during supervised visits he can control himself, visiting restrictions are weakened. Eventually the plan is always for normalcy in this parent-child relationship, which would mean custodial care or at least unsupervised extended time together.
And even though hitting an infant is not even accepted in "mainstream" parenting, as your baby grows in the months to come, a greater and greater percentage of people-- including judges, social workers, etc.-- will see the practice as more and more "normal," even if not ideal.
So I do think leaving, if for this reason alone, is a decision that should be considered with great caution.
That said, I wouldn't leave my child alone with him ever, at all, for even a few minutes. And that is a terrible burden for you to face over the longterm. Hopefully in the meantime you can influence him to seek some therapy and parenting classes. I encourage you to think about what approach you might take in that regard...to what kind of approach might he be most receptive?