The North Korean novel "Jackals" from 1951 by Han Sorya provides an MO for what might have happened to Otto Warmbier.
In the novel an American missionary is showcased injecting germs into a Korean boy to kill him. You can read about it here.
I don't think it's a coincidence an American visits North Korea and suffers a similar fate at their hands. I'm guessing the North Koreans actively gave something to Otto Warmbier which they knew would kill him eventually.