Poisoning the grove doesn't help the protagonist in the slightest. He doesn't get the favour of powerful people, he can't really expect a reward bigger than the one he gets for not poisoning it, and it isn't really much easier to do it that way. So the protagonist must either genuinely want to help the halfling, or really hate the druids.
So, the only people who would poison the grove are psychotic madmen (who are in it for the killing, not the money) and, interestingly, good people.
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