Whoops! Your link goes back to your comment... here is what you want: http://www.huffingtonpost.com/2012/05/19/marissa-alexander-gets-20_n_1530035.html
Even so, the sentence is way out of proportion here, which you acknowledge. But it bears illumination, I think. If she was the victim of repeated abuse, it seems likely that she wasn't thinking straight and maybe intended to show him she meant business. Law enforcement kind of has a history of blaming women for their own abuse, and I doubt those biases have been exercised from the thin blue line. There are also cases upon cases where women did just what she did, took out a restraining order, only to be stalked, beaten, and murdered by those who claim that they loved them. Plus, this is a black woman to boot, who historically have been under both racial and gender oppression, with the two ideologies denying her the white woman's "privilege" putting her at greater risk for abuse from the system of racist and sexist oppression. Rarely were the abuses heaped upon black women taken seriously, in other words. I doubt any of this was far from her mind as she acted, to her mind, to defend herself and her child.
And have you ever sent someone to your house to find/get something for you? It's far easier to do it yourself, no? But it seems she went to what was once a shared house, with the expectation that he would be gone... she did not go to confront. And kid/s were present, it's not a stretch to think a man who would beat a woman would do the same to a kid... If she came back in with a gun, maybe she intended to warn him away from doing that?
I'd argue that these two cases do indeed reveal the uneven ways in which "justice" is doled out in this country, and they tend to favor the white and male, if not those with more resources to throw at the problem. This doesn't mean the same sort of openly racist trails we saw well into the 60s, but rather more subtle means of structuring privilege, including the passage of laws such as this.
Whatever the source of the disparity, a woman is gong to jail for having killed no one, and a man is walking free after having shot a teenaged boy. This hardly seems just.