What do you do career-wise? It sounds like with the introspection you have been doing it might be time to try and make a fresh start somewhere else, but that will depend on what you can do to support yourself.
As far a guilt and regret goes, it may be worth the time trying to reach out to the people you have hurt the most and, if nothing else, apologize. It helps you more than it helps them, and there is maybe potential to make peace with some people.
Forgiving yourself is harder, but there is only so much you can do about the past. Repair what you can and learn from what you can't.
More likely 10-15% of people die then everyone figures out it's the water, identified the cause of death, develops filters to remove the toxin, and then the filter becomes commercialized.