He refused to cure a developing species of aliens essentially suffering from being on the bad end of natural selection, thereby establishing the ethical precedent that would eventually lead to the prime directive. In other words, he's a badass.

He failed to give a cure to a genetic error to the Valakians so the Menks could eventually stop being slaves.

Not only that, but he was an hypocrite for deleting it afterwards.

And sure, using some shadily obtained data to treat B’elanna was ok, but somehow restoring two people to life by correcting a transporter error was unethical? Such hubris