In her
statement that was made public she claimed that "she served the meal and allowed the guests to choose their own plates and then took the last plate and ate a serving herself."
Which is kind of a weird way to serve Beef Wellington if you think about it. I think the most common way to serve it would bring it to the table, carve it in front of the guests and hand each person their plate.
However, doing it this way allowed her to suggest that by picking the last plate she couldn't have possibly known if she'd get a poisoned serving.
I wonder if part of it was the "illusion of choice", like you see in magic tricks. Most simply in this instance that could be: the choice of plate with the Beef Wellington ultimately didn't matter because the poison wasn't in that part of the meal.
As some of the wonderful comments have mentioned up thread, poisoners often think of themselves as smarter than those around them, so it wouldn't surprise me if she a) picked a Beef Wellington in the first place because of the duxelles and thinking she was oh so clever with the deliberate red herring, and b) loved the feeling she was tricking the guests with the illusion of choice.
With what we now know, she had likely tried to murder Simon multiple times and failed... she was probably wary of both Simon becoming more suspicious of her cooking, and the fact that this intervention meal was possibly her last chance of getting
easy access to his food. So this illusion of choice could be super important to first try and lower his guard enough that he
would even eat food she prepared, and secondly to try and delay possible suspicion when he
did start to get sick, so he(/they) wouldn't raise concerns until it was too late to communicate again. When he cancelled, it may have caused immense rage that he apparently got spooked (made her feel like she wasn't as sneaky as she believed she was?), and that her final, Hail-Mary plan was thwarted.
We know one of the three that died did manage to have a convo with the paramedic that made them alert the police. Wonder if it was one of his parents, who he may have shared his fears with - even if in the broadest terms? If so, it may be that "paranoia" was the reason one of the victims was able to make the connection and alert the paramedics, even before Simon even got to the hospital, got his bearings and made himself heard. Now it may be deathcap poisoning is so specific that it would've never gone under the radar, but this was a woman was likely emboldened (and frustrated) by the previous attempts that hadn't kicked up too much dust.
IMO.