It's interesting to note that while Antimatter and Purple Nymph invert a creature's attack, there are no cards in the game that decrease attack. There's no way to know for sure whether or not Zanz would allow attack decreases to go into the negatives, though I'd bet most of us would lean towards yes.
Huh, that is interesting. I wonder why
There's a sort of unspoken rule that you have to follow the rules of the game unless you're intentionally and clearly breaking them.
So if there's no way to decrease attack, there's no rule that says you can't make them go into the negatives.
It's like how CSGO gambling is legal because the lawmakers haven't outlawed it - no rule to punish them, so they go free.
It's worth noting that every single attack reduction idea I've seen states you can't go into negatives, because going into the negatives is stupid.