What if they're killed simultaneously?
Ex. 2 Hydras on the field, both with 3 hp left. Opponent uses rain of fire. Do you have 4 hydras or 0?
You would have 0 left. That's part of the strategy with this card. Your opponent would have to kill all of the Hydras simultaneously in order to wipe them off the board. So having just one Hydra out there is essentially useless. You'd have to play two at once, OR if you're anticipating a Rain of Fire or some other card that attacks all creatures, play one after another to try and get them at uneven health so they die on different turns.
I think that way it presents interesting strategy questions for both the player of the card and the opponent.