You can make yourself a better person and lead by example, but you can't stop people from doing something like spreading misinformation and hate speech. It has been going on since the beginning of civilization. If your startup has the right moral and socially beneficial intentions, then that is really the most you can do. If someone comes along and uses your tool for evil, that doesn't mean you shouldn't have created your tool.

In scientific and technological research, there are often amazing uses that benefit humanity. For example, splitting the atom resulted in nuclear power which is arguably a great invention for mankind. However, due to human nature, it created the greatest weapon mankind has seen. You cannot blame the researcher for facilitating this, you can only blame human nature for abusing the technology.