Are most homicides committed by women due to domestic violence?

In the case of marital homicide, yes. A significant proportion of the killings by women are acts of self-defense. Almost none of marital homicides by men are acts of self-defense. Female homicide victims are more than twice as likely to have been killed by an intimate partner than are male homicide victims.

And sadly, when a woman is murdered, when the police "round up the usual suspects," to use that classic line from the movie Casablanca, the first suspects are husbands, boyfriends and former husbands and former boyfriends.