I heard somewhere that Zanz believes that any insta kill is too powerful for one card and that he doesn't really plan to implement any instant-kill cards because they can easily be game breaking. (Or something along those lines, I could be wrong, but you get my point)
Mutation doesn't count because it's a measly 10% chance and the fact that's usually not abused as creature control, and shockwave, it requires 2 cards to pull off the combo anyways.
In other words, this card is OP. Just make it do damage.