I'm sorry about your ex-boyfriend's behavior. That must have been insanely frightening to you.
Yes, she may have killed her baby and not remembered it later. In that case though, one would have to accept that she not only killed her baby, but also cleaned up the crime scene, disposed of the body where no one can find it, then returned home and went to sleep - all of which was then blacked out. It's theoretically possible, but would be highly unusual.
Alcohol does lower one's inhibitions and when that happens people act upon impulses they normally can restrain. When sober, your ex-boyfriend may have been able to restrain his impulse to behave violently, but with his inhibitions lowered while under the influence of alcohol he was no longer able to control those impulses. The same may be true of DB - while sober she could control herself around the baby, but when drunk was not able to restrain a momentary impulse to shake her or throw her or strangle her.
Many, if not most, violent crimes are committed under the influence of alcohol - especially domestic violence against spouses or children. Alcohol does not "make" people violent, it makes them unable to restrain their violent impulses.
Many aggressors claim to be "sorry" afterwards, and I believe many of them are. We may be seeing some of that with DBs behavior now. Whatever happened that night, she many be feeling very guilty and sorry about it.