The mirrored abilities show that he needs the antidote to his poison for an assassination attempt and that his opponents can benefit from the antidote as well if they eat it. What do you think?

Fascinating concept. Nice and flavorful. I like it. Not sure he should have 4 toughness though. Would changing him to a 2/3 make him too weak for the cost?

Probably, as Lightning Bolt and friends are very prevalent everywhere, but 4-damage burn spells aren't nearly as common. Perhaps a 2/3 deathtouch and lifelink?

That would be excellent. One keyword from each color.

Since the player is casting this for 2BG, we already know that the player can make black and green mana. This means the riders to the abilities will almost never be relevant and may just be taking up space. In practice, this will pretty much always play as "1BG, T: Destroy target creature." as repeatable destruction, this ability is also quite undercosted. :)

Fair point. Do you have some thoughts on ways to fix it?

You're right... Maybe "sacrifice unless you've cast a green spell this turn" and "oppt. gains 2 life unless you've cast a black spell this turn"?

Ooh, even better. Sounds good.