Criminality in the UK requires mens rea (intention) and actus reus (an actual act or omission). You have to have both.
This means that people who intend to hurt someone or something but don't actually do it, they aren't criminals (ie thinking how nice it would be to brain the noisy neighbour isn't criminal, until you actually do it).
Also means that people who hurt someone unintentionally aren't criminals (for example if I dropped a brick on your foot - it would be an accident).
Of course that doesn't mean that the brick dropper can't be indicted for something else, like negligence for the brick dropping etc.
So I think they can't be done as criminals, because they didn't intend to hurt anyone and had measures in place for what to do if people deteriorated. Also it's a medical trial, so there are risks inherent in that.
Mens rea just means mental state, it doesn't have to be intention. You don't have to prove intention for manslaughter, recklessness will do. Negligence will do for other crimes. There are even some crimes of strict liability without a mens rea where you don't have to prove any mental state, eg possession of drugs (this was true when I studied law in 1984, I don't claim to be current).