I solved this one in the process of asking the question, so I'm self-answering.
I remember it as being part of an Alfred Hitchcock anthology. The protagonists get pulled into investigating the murder of an old lady who's found dead on her kitchen floor (I remember very vividly the black-and-white drawing of her contorted body lying there) with no apparent motive for murdering her. It is found that the hamburger that she ate that night was poisoned with strychnine. Eventually, the protagonists determine that the actual intended victim was her dog, who she had written into her will. The killer had seen that she had bought steak and hamburger, and poisoned the hamburger, assuming that would be the dog's food, but she'd given her dog the steak and eaten the hamburger.
It may have appeared in the same anthology as "Yours Truly, Jack the Ripper" by Robert Bloch.