For context: I’m writing a Western novel but still in the early stages of planning out the plot; and one of the characters is a detective investigating the death of a man in the year 1899 (May change to 1900).
The deceased man is examined by doctors who determine that he had too much insulin in his body at the time of his death and without any other way of looking into it, they say that it must have been because his pancreas failed, which in turn caused him to die.
As it turns out, the detective finds a small mark on the corpse that appears out of place since it is located in a weird spot (under the toenail). He’s curious of foul play and as it turns out, he was injected with insulin in his sleep which was easily covered up with the pancreas failing explanation.
My question is: how realistic would this be for the given year? I know insulin shots were not officially done until the 1920s, but given the technology at the time, could this bizarre event theoretically happen? The ‘antagonist’ behind this is a wealthy man who invests money into various research.
Insulin would not be discovered until 1910, by Sir Edward Albert Sharpey-Schaffer, when he concluded that it was some hormone produced in the Islets of Langerhans in the pancreas that controlled diabetes, and named it insulin. But even after that, no one would manage to actually isolate insulin until Frederick Banting, Charles Best and John MacCleod in 1921, after there had been unsuccessful attempts to control the disease by simply grinding up Islet cells and injecting them.
Well, kind of yes, but also kind of no.
First of all on 1899/1900 Insulin wasn't discovered yet. Scientist already figured out that you need the Pancreas or else you will develop Diabetes (1889) and they also figured out that you could treat Diabetes by injecting an extract made from the pancreas (1894).
The name Insulin was only coined in 1909/1910 and only in 1916 scientist were able to isolate Insulin.
So you could definitely make an argument that it is in the realm of possibility that in 1899 someone could artificially lower the blood sugar of another person by injecting them with pancreas extract. I find it highly improbable however that one would have been able to actually murder another person like this, considering that the concentration of Insulin in the entire pancreas is not incredibly high. You would have to inject a significant amount of Pancreas extract, certainly far too much to be able to inject it below someone's toenail.
And then you'd be running into the next problem: In 1899 there was no way a doctor would have been able to figure out the cause of death in case of a insulin overdose. They simply couldn't measure blood sugar. Measuring blood sugar is something that was only developed in the 1960s, before doctors diagnosed Diabetes by measuring glucose in the patients urine. This has been done since thousands of Years, simply by tasting it (Diabetes mellitus translates to "passing through honey sweet" basically transcribing both the increased urin production and it's sweetness). Unfortunately they could only figure out if glucose levels in the urine were too high, simply by the fact that a healthy person's urine does not contain any glucose at all.
So to summarize: Yes, it's not entirely implausible that in 1899 someone would be able to kill a person via what's basically an insulin injection, but a)it's already very unlikely, b) nobody would use the term insulin and c) it's impossible that an investigator would be able to figure out what has happened.
Small addendum: A failure of the pancreas would not lead to excessive insulin levels. So even if doctors would have been able to figure that out, the conclusion would be outright wrong regardless.