Right now, thematically speaking, I was thinking that permanents that are destroyed or removed by force. Aka, the explosion of an unstable gas = poison, while the disappearance of a sundial or phase shield does not.
That leaves animate weapon, with the radius argument, that isnt really a problem because for the most part, you would have to place quite a few permanents before it would protect your weapon. If i have blast spores protect all permanents, then technically animate weapon would be a source of poison, thematically speaking.
As for whether it should always hit the opponent or not, Its meant to be defensive not offensive, but im open to everyone's opinions.