It's actually quite striking how close the figures once were. I can accept that less women are resorting to murder since social support exists to help them exit a toxic relationship.
Now, something occurs to me. The noted lack of support places for male abuse victims. The disparity between crisis shelter places for the two genders is over 100:1 in the UK for example (whilst the abuse prevalence is roughly even), which is an indicator of the difference in how seriously it's taken. If we hold that support structures and crisis shelters for females have reduced female killings of their male spouses, then it's entirely possible that if female->male abuse was taken more seriously, and social support was in place for male victims, we might actually see a reduction in the deaths of female spouses.
After all, when support places for women didn't exist at all, the spousal murder rate was almost identical between the sexes. And take into account, men and women who murdered their spouse tend to use the same justifications. "he drove me to it" => "she drove me to it". Are we to always believe the woman, and always call the man a liar when they say this? Women disproportionally use poison as a murder weapon, rarely a gun or a knife. If you poison someone, that's shaky as "self defense", and it fits the pattern of "breaking a cycle of abuse" rather than "fighting for your very life" - women's weapon of choice doesn't correlate well with the self-defense claim. And either gender could be locked into a "cycle of abuse" and want to end it. An abused man, after all, may have nowhere to go (since there's no social support for him), and an abusive woman can definitely threaten him with losing the house, access to kids, false accusations etc., to keep him there until he snaps.
It's not sexist to say this, since we just established that social support for females likely reduced the male death rate. Unless you want to hold to an orthodoxy that one group is always at fault, and the other group is always to be given the benefit of the doubt. I suggest you read the link Vector provided.