Over the last decade we've received hundreds (thousands?) of stories of men killing their wives and/or girlfriends. In recent years these are often flash fiction pieces where the characters are introduced and then, bam!, the woman's dead. Sadly, after reading these I think, "Eh." I've seen it before, and, frankly, I don't care about any of the characters. And thinking up a unique way to kill off the woman doesn't help.
Consider bucking the trend and writing a macabre piece that doesn't involve murder. :)
In your story if you want to kill off your woman, make me care. How do you do this? You create a three-dimensional murderer and murderee, with good and bad characteristics, loves, hates, and desires. Real people want something(s) and so should murderers (and I'm not talking just murder). The murder should arise as a consequence of their characters. The protagonist's character should not consist solely of murderer.
Is this hard to do in flash fiction? Yes. Very. It's difficult to do in 5,000 words, too.
But no one ever said writing was easy. Or, if they did, maybe they deserve...