Examples of where medical information from the population could be used for bad purposes:

• Keeping a database of people who have had abortions so you can retroactively arrest them when you change the law.
• Keeping a list of trans people so that you can find out who is partaking in illegal private healthcare.
• Searching DNA for genetic markers for autism so you can "cleanse" the world of them.

I understand the appeal of thinking that with all this medical data we could help the world and make positive change, but I don't think that's the world we live in. I can't speak for everyone, but I think most people are generally willing to give you their medical information if you can demonstrate to them that you'll use it to make the world a better place. Whereas if medical data were taken by force without consent, that suggests that you don't value your patients' agency and perhaps their other rights.

(Disclaimer: Some of these may not be accurate to real world plans (I try to avoid looking into drama too much), but at the same time it's believable to me that a country somewhere could go for them as described)