Certainly, you can kill the wife. Women are killed all the time in fiction. Everyone will expect it. To get sympathy for your protagonist, make sure your reader knows she was a faithless virago. That will sway their small, judgmental minds. How dare she stray, break the marriage contract, and still ask your protagonist to shower daily?
Stack the deck by mentioning her intention to go through with a brutal divorce, to take the child, the house, and half of the priceless Funko action figures your protagonist has slaved for years to collect. That’s your stuff—your MC’s, I mean.
You can kill off her lover, of course. No one will miss him. The athletic type with the full head of great hair your wife meets cute on that so-called “Girls’ Trip” to Vegas. Describe him as a leering Lothario who disrupts the happy home of your, let’s say, moderately successful artist hero (not a writer though, no one buys stories about writers anymore).
Now: The child. The child is a dilemma. But he comes in at the wrong time, catching you catching your wife in flagrante delicto, and now there is blood everywhere, and he’s crying. Loud as a siren. You will lose the reader’s sympathy if you off the offspring.
Unless…you describe him as a problem child, spoiled rotten, a demon seed—possibly not even from your own loins. You’ve tried to love him, the stinker, tried to be a good dad. But there is that time he pushed me—I mean you—off the ladder. He did! Ten stitches! That time he poured sippy juice all over my—your—keyboard just as I had got the leash on a story I—you—struggled with for months. Those moments don’t come easy. Sayonara, junior, and good riddance.
But the dog. You can never kill the dog. Even in a moment of passion. Everyone knows that.
It doesn’t matter that Coco hates you, going out of its way to sneak into your office, jump onto the desk, and poop copiously on your last manuscript.
It doesn’t matter that it is the dog who has gone along on the “Girls’ Trip” to Vegas and is the one who has first approached the leering Lothario, wagging its suggestive tail and practically tripping him to get his attention. You know this, of course, because the dog has told you, whispering into your ear as you napped together.
It doesn’t matter. Because everyone loves a dog.
Even you. Despite its animosity and criticism of your work. Just look at it. Wet nose, big rheumy eyes, cutest little underbite.
Kill the dog and you lose your soul. Worse, kill the dog and you lose the jury—the reader, I mean.
Well, you lose anyway. You lose everything.
And so, you spend your days curled up in a ball, flea bitten, bored, waiting for chow time, and restless after lights out. And the only writing you do now is long, rambling letters to Coco, letters that poochy-coo never even bothers to answer.