I am sorry I am no expert at killing people but like..... if you really wanted her dead why not use a shard to slice her throat or bring a knife for gods sake. Why would you choose to burn her, inside a mansion nonetheless. Like, wouldn't people be able to smell the smoke?
Also burning alive is a rather long death and even if, why not pour stuff on her and then set her on fire, instead of everything AROUND her.
I mean if you want her dead, why be so ineffective. Unless killing her isn't the main point and it's a demonstration of sorts.