A defence barrister will try to demonstrate that the chain of causation for alleged murder has been broken.
This is because there needs to be mens rea, actus rea, and an unbroken chain of causation for a murder charge to be fulfilled.
Breaking the Chain of Causation
Novus actus interveniens is the Latin legal maxim, meaning that there is a new intervening act (or omission) alleged to have occurred, (by others), breaking the chain of causation.
The chain of causation may not be alleged to be absolutely broken, it can be argued that others contributed to it being broken, eg by action, inaction, unconscionable delay.
Such contributions could be seen as medical accidents or negligence.
The others who legally may be alleged (by the defence) to have broken or contributed to the breaking of the chain of causation can include other professionals (and even an Act of God, in some circumstances).