A documentary about Unit 731 in r/Documentaries sparked my line of thinking below and I hope someone here can assess the likelihood of it being true. An answer along the lines of, "No way, that's not that likely to occur or haha, you're insane" would be best.
My grandfather (father's step father) was a saint. He was a doctor who escaped China to Taiwan when the communists came to power. When he married my grandmother, he provided for her 3 children from a previous marriage and his cousin's children. My grandparents would also raise money for orphans. They operated a hospital and he would not charge the poor for care (good luck finding a doctor like that today).
My grandfather grew up in Japanese occupied Manchuria. His good friends were Japanese and he went to medical school in Nagasaki. Here come the red flags I remembered watching the documentary: Years ago, when I asked him about his past in China his reply was, "I can't tell you about that. It was too terrible and sad." He said he left Nagasaki 3 years before the bomb. That would mean he started medical school before the start of WWII? Did Chinese regularly go to medical school in Japan at the time? What kind of Chinese would do that?
He left China in a big hurry. He left his wife and three sons and only had time to hug his eldest son before he got out of dodge (I always thought this was because people would view him as a collaborator or, in hindsight, maybe he was). He was a firm atheist. His words, "All we are is meat. After we die there is nothing."
Given the timing (when he left China for Japan, and vice versa), his beliefs (there is no God because he saw/abetted/did some horrible shit maybe), and actions (help as many people as he can to atone for the horrible shit he saw/abetted/did maybe), please disabuse me of the notion that the relative I love the most was recruited into Unit 731.
TL:dr Hoping my grandfather was not a local collaborator with Unit 731
Firstly, Unit 731 wasn't that big. There were a lot of collaborators who had nothing to do with Unit 731. Policemen, regular doctors, civil servants, etc. The Japanese ruled, if you wanted to be successful you had to go along with them. If he was friends with the Japanese, went to medical school in Nagasaki, then it's very unlikely he was an anti-Japanese resistance fighter... but that does not mean he was working with Unit 731. There were hundreds of thousands of people who accepted Japanese rule and did the best they could, without committing human vivisections, injecting people with diseases to see what would happen, exposing prisoners in the snow to see how long they'd live, and 731's other atrocities.
Do you know what city in China your Grandfather lived in? Unit 731 was based in Harbin, a regional capital.
Secondly, do you know the ethnicity of your Grandfather? Was he a Han, Manchu, or something else?
China was ruled, under the Qing dynasty, by an ethnicity known as the Manchus (whose homeland of course, was Manchuria). The Qing Emperors all came from this ethnic group, and Manchu had special privileges. When the Japanese invaded, they continued to use Manchu as civil servants, etc, in Manchuria, with the Qing Emperor as a puppet ruler. Besides the terror of Japanese soldiers, many Manchu (and other Chinese) obeyed the Japanese-controlled puppet government because it was still the government of their Emperor, while the rest of China was ruled by Nationalists who'd overthrown the Emperor.
What I'm saying is that even if your Grandfather worked with the Japanese occupiers, this does not mean he committed atrocities. He could equally not want to talk about the period because he saw atrocities (either by the Japanese or the Communists when they invaded Manchuko and took it back from the Chinese). Or it could be that he is ashamed of being a collaborator, but not because he did such bad atrocities. I have a friend who has 3 uncles who were Manchu policemen under the Japanese (they all changed their names to Han names after the Communists took control). Yes, they were collaborators, but that's not the same as taking people to be experimented on.
It should also be said that, while locals were needed for brute labour, the scientists of Unit 731 were Japanese, not Chinese.
I'd also say that the Nationalists were not okay with Japanese atrocities. They fought the Japanese for years, and it was the Nationalists who controlled Nanjing when it was invaded by the Japanese who carried out such brutal massacres. So, while it's quite possible that a Chinese collaborator could hide their crimes from the KMT, I wouldn't say the KMT would be okay with it if they did know.
One more clue (?): my colleague just pointed out my grandfather escaped China when the Communists came to power. Meaning he was ok under the the KMT even if viewed as a collaborator. It is my understanding Unit 731 has a thing for experimenting on communists.
It would have been extremely unlikely for your grandfather to be involved with Unit 731. There were 53 actual doctors (as in, with a medical degree) in the Unit, a partial list of which can be found here. This list was put together by Waichiro Omata, a Japanese medical historian who previously had work comparing Nazi war crimes with those of Unit 731.
In this list there are about five names that pick up a hit for Nagasaki Medical College. The first two, #52 and #53, are professors (of pathology and physiology, respectively) and have very Japanese names. The third one, #63, is a senior official at the university. The fourth one, #92, is an emeritus professor and would have been 40 by the end of the war. The last one, #133, was a senior lecturer in Kyoto Imperial University by 1929. Presumably none of these fit the profile of your grandfather as they are all much more senior than he would have been fresh out of school.
It was certainly common for Chinese students to study in Japan, although this was primarily before the Sino-Japanese War started. There were thousands of students in Japanese universities at the start of WWII, many of whom were from Manchuria (people of Chinese descent in Manchuria fell into the same bucket as anyone else from Manchuria in the Japanese system as of 1938).
As to why he would leave in a hurry-the Soviet Union, after invading Manchuria in 1945, turned over the territory to the Chinese Communists rather than the Kuomintang. And they certainly took a dim view of Japanese "running dogs", at least more so than the Nationalists.
Lastly, it is important to note that the Japanese really did not want people to find out about what they were doing at Unit 731. Unit 731 was only established in such a manner after the original fortress where they conducted experiments had a prison breakout. They demolished the facilities in order to try and cover up their actions. Given these previous actions, the idea that the Japanese would trust any foreigner to hold their silence-especially one who was from the local area-is somewhat absurd in this regard.