I read this in 1970 I believe. Involves people in the not too distant future being able to buy chemical kits to create an animal for a special purpose. In this case a woman is trying to kill her husband (not clear if he is abusive or not) and buys a container of "attack dog" to put in the house to attack him when he comes home. When creating the dog, she spills a few drops and the result is the dog ends up lame and not complete so the husband is able to kick it to death. I think there are a few more unsuccessful attempts over a period of time and then at the end of the story, the husband corners her and says he found out about the attempts and is going to fix the "problem of their marriage" by using an "old fashioned" method a friend made for him, a large butcher knife.
I think this is the one. Harlan Ellison's Do It Yourself
Here's an abridged excerpt:
“This is your own Do-It-Yourself Murder Kit,” the pamphlet broke her literary reverie with harsh reality. “The new guaranteed Murder Kit, with the double-your-money-back warranty, for your protection.”
Well, she thought, frugally, that’s nice, anyway. That double-your-money-back thing.
“This Kit contains three sure, clean and undetectable, I repeat, undetectable, ways to commit murder. No two kits are the same, through repetition occasionally occurs where the subjects to be murdered have common character traits. Again, though, no two kits are the same. Each of the three modus operandi is designed for you according to the application blank you sent us when you contracted for this Kit. Now. To prepare yourself for your murder—”
She snapped the pamphlet shut with quick, suddenly-sweating hands.
Do I hate him that much?
She answered her question firmly: Yes, yes, I hate him that much. And more!
She opened the pamphlet again. Her hands had become dry and almost cool again.
“The first method of murder we have prepared for you,” the pamphlet’s voice continued, undaunted, “is the rabid dog method. You will notice a coil of grey substance. This is your Animaux Tube. Warning is issued at this point that instructions throughout the use of this Kit must be specifically followed, or failure will result. There is no mechanical failure possible with this Kit, only human failure through inefficiency and disregard of stated operating procedures. Is this understood?”
“Yes, I suppose so,” Madge answered, surlily.
and here's the ending:
He smiled again… this time with dirty mirth. “…your goddam floor’s gonna get filthy today, Madge.” He drew out the long, shiny knife. “Had one of the guys in Steel Molding make this for me… a real do-it-yourself.”