When Charles and Oliver find Mabel covered in blood and kneeling in front of Bunny’s body, she says about the now-deceased Arconia board president, “I just opened the door and she stumbled in on me.”
Between Bunny “stumbling into” Mabel’s apartment and having been stabbed with one of her knitting needles, it seems likely that whoever killed Bunny was trying to point the finger at Mabel.
But what if the knitting needle was just a weapon of opportunity, and Bunny just happened to end up in Mabel’s apartment? It’s a stretch for sure, but not impossible, since they live on the same floor and Mabel’s might have just been the first one Bunny reached.