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."
Perhaps his decision to make it not instant kill was BECAUSE of the malignant cell thing. If that was his focus, perhaps he just wanted to introduce the malig cell only card so that when people saw it they would think
"OH WOW! That card spawns Malig cells"
and not
"OH WOW!, Not only does this card kill instantly, it ALSO does malignant cells."
The statement is vague enough to have infinite interpretations. Ultimately Zanz never said "I dont want instant kill cards" He just said he didnt want the card to spawn malig cells AND be an instant kill card.
EDIT
Also, lots of times when a dev wants to add something new to a game that they are really proud of, they try to make it TOO simple to get, so Zanz's mindset could have been, alright, I made this card, and this is the fastest way to see it done. And then all of a sudden think when you brought it up "Oh wait... this is a pvp game, and this card disrupts the balancee... I like showing it off, but perhaps thats a little too much. Instant kill = bad FOR THIS CARD. "
I know Ive done that before. I have been excited about finding out how to implement something and then just do it the fastest way possible and think about how it effects the overall game later on.