A tragedy, caused by negligence - fortunately a rare type of incident, but rarity is immaterial when it happens to your family.
The report mentions syringe - a syringe that measured in ml, rather than insulin units was used. So, the insulin was probably drawn up from a vial, rather than using a cartridge in a pen device.
The aim with administering medication as a nurse is (in no particular order) right patient, right dose, right time, right medicine, right route - in this case, the wrong dose was administered, because the wrong syringe was used, so the dose could not be measured accurately.
Those with long memories may remember that in the the 1980s, insulin changed from 40 or 80 units per ml to universal 100 units per ml - not all countries changed at the same time, so replacing a loss of insulin overseas was even more complicated.