Laura - was she an avenging angel??

I'm about to spoil this film big time so if you haven't seen it, look away. The last scene shows the first Mrs. Harrison's (Edith's) loyal servant Laura smiling with satisfaction as the body of the new Mrs. Harrison is hauled away, as is the very much alive but wounded Mr. Harrison - destined for the gallows for killing the second Mrs. Harrison (Kay).

Now Laura had served Edith since childhood, so she knew where everything was in that house. Did she set up the situation that occurred to get justice? She knew what was in Edith's diary, she knew where it was, and she managed to keep around the one poisoned bottle that showed Edith's claims were true. She planted a seed of doubt in Kay's mind knowing that Kay probably just wanted Ralph's money, but acted unknowing - claiming she didn't know where the diary was and that if ever found it might implicate Ralph in Edith's death, and that the key bottle of Edith's medicine was never found.

Yet Kay, newcomer to the household, easily finds both the diary and the poisoned bottle that have supposedly alluded Laura. I don't buy it. I say Laura set them both up because she read the diary and thus knew Ralph poisoned Edith's medicine and thus intended to kill her, yet the last page of Edith's diary proves Edith knew the bottle was poisoned and didn't take the medicine - dying of natural causes a short time later. Thus taking the diary or the bottle to the police would have done no good, so instead she set Ralph and Kay up to turn on each other. What Laura couldn't know is that Ralph would pick up the phone and hear Kay plotting. Did Laura perhaps pick up the phone and hand it to Ralph, just pretending to be the good servant she had always been? Or perhaps she said things to Ralph to make him doublt Kay just like she said things to Kay to make her doubt Ralph. I'd like to think so. What do you think?