If there is some unwritten rule against "instant kill" cards, then how did an instant kill card make it past designing, coding and alpha, all the way to beta?
Simply because Zanz was so focused on the malignant-cells aspect of the card and getting that part right that he literally never stopped to notice that it was an instant-kill card until I mentioned it to him. When I did, he essentially replied with "Oh...wow, I guess you're right. That'll have to change."
Why would it 'have to change' if there wasn't a specific intent on his part to avoid instant-kill cards?
Mutation is not an instant-kill card. It's 40% of an instant-kill card with some potentially very bad repercussions if the kill part fails on an enemy creature -- so much so that players often voluntarily play it on
their own creatures (which you would almost never do with an instant-kill card, Antimatter being the exception that proves the rule).
Aflatoxin isn't an instant-kill card anymore, because instant-kill cards weren't part of the designer's intent at the time.
Shockwave can be an instant-kill card -- again making it the exception the proves the rule -- but the specific convolutions you have to go through to make it work as an instant-kill card are profound and often pointless.
Conclusion: Designing an instant-kill card and putting it in front of Zanzarino is going to be a waste of time in 99% of cases. Thus, the unwritten rule: no instant-kill cards. Can you go ahead and try for that 1%? Sure. Just do it knowing that your chances of any form of recognition are slim-to-none.