It is so easy to put some insulin into an i/v bag. Or anything lethal, really.
I believe she is guilty.
When Allitt was charged it was because she was the one person who was THERE for every single critical incident - and she had a terrible attendance record and sickness record - she injured herself on purpose to get attention when she was s pupil nurse.
Also - there had been many incidents of her "getting even" with people who had incurred her displeasure - an old boyfriend, for example. And there had been very disturbing things going on in the nurses' home - faeces left in the fridge, e.g.
I am fully expecting things like this to come out about Letby now.
This is the definitive book about Allitt:
https://www.amazon.co.uk/Murder-Ward-Four-Biggest-Criminal/dp/0701148136