You’ve never dealt with the shailoh of how we can hurt ourselves if everything is min hashomayim?.
Take a look at rav avigdor miller’s hashkofa seforim, chovos halevavos shaar habitachon, and rav chaim friedlander (who presents the issues very systematically). The upshot is that for instance, playing in traffic on ocean parkway is a sin, so the person is punished for that sin by having the damage dealt ro him.
If someone attempts to poison someone else …here it gets complicated. No one can hurt you without it being a gezerah, ain odom nokev etzbo, etc. Yet a murderer is punished, even though no one can die without it being decreed on rosh Hashanah. The answer is that Hashem considers it as if you did it, because of your intention or negligence. This is how teshuva undoes the sin – it takes your portion, meaning your intention or negligence, out of the picture, and all that remains is the maysoh shomayim in the effect.
How a baal bechirah can affect you is discussed a lot by rav chaim friedlander. He says that his bechirah can bring you into a din torah in shomayim where they’re dan you in a stricter way than normal; he’s meorer dinim on that person by trying to hurt him.
Hence, poisoning the water can only affect others if it was decreed so , especially when it’s a rabim and not an individual. The poisoning would have happened without you, but you are held accountable for the ill intent.
When it comes to what the neviei sheker postulate… they’re not authoritative in deciding what’s a peshia/negligence. They’re political hacks who have no idea how the world works.