In all fairness, it seems like the "burning her to death" part was intended to be their way of destroying the evidence. She was probably supposed to die/be permanently incapacitated by the poison, and then they'd set the fire to cover their tracks and maybe finish the job. This was never Plan A.
I can see why they wouldn't want to improvise stabbing her: blood is extremely messy, the wounds might be discovered if someone examines the body later, they may not have prepared a knife and shards of pottery don't make super good weapons, she's still conscious and will definitely struggle, and so on. It's not totally outlandish to just skip to the fire part of the plan, if they assume that she's just going to die anyway.