Drugs aren’t controlled because they’re dangerous - e.g. insulin - but because they’re desirable, mind-altering and addictive. Opiates, barbiturates, that kind of thing. At my last Trust you didn’t even need two nurses to give a controlled drug. As a CNS visiting wards I was just handed the keys and invited to give my patients morphine/whatever. As it wasn’t what I was used to I always found someone else to check it with me. Protocols vary from Trust to Trust, but nurses are accountable for every single thing they do. We can on the whole be trusted with drugs.
Nurses have to know the correct dose of every drug in case it’s been wrongly prescribed or dispensed, because we are accountable for our actions and cannot blame the prescribing doctor or dispensing pharmacist - we gave it when we shouldn’t have, entirely our fault. We have to know the side-effects and contraindications of every drug we give, and its reactions with other drugs. And of course many nurses themselves are now prescribers following specialist training.
There are dozens of different drugs on the average medical ward drug trolley. Most/many of them would kill a person at a high enough dose, or in the right (wrong) combination. Nurses could easily kill themselves, colleagues or patients whenever they felt like it, but the overwhelming majority wouldn’t dream of it. I hate having to reassure people of this. The Letbys and Allitts are anomalies, incredibly rare, and always shocking.