Good thing this was posted, as I wanted a more thorough discussion about this.
I believe that kill spells balance out at some point before it becomes a 1-for-1 trade against a target, taking quanta cost into account. The main use of single-use control spells in the game is an advantageous quanta tradeoff (2
for a 10-cost dragon, etc), though with the added value of potential removal of a combo. The downside is that there's a chance that you might not be able to get a good tradeoff, like if targets are cheap, or if there are no targets at all. A vanilla instakill card would also meet the same problems, so the only question now is when that 1-for-1 trade balances out with the problem of not being able to do advantageous tradeoff. For sure that's below 10 in a mono setup, and I'm leaning to agree on serprex's 5, or maybe 6 if we're being conservative. And we have to face that Elements is not as combo-intensive as Yu-Gi-Oh: stopping a world lock with minor control will definitely win you the game, but destroying a momentum-adren staff does not exactly have the same impact as there may be 4 or 5 more in the deck.
Only problem I see now is that it depreciates HP, like what Ginyu had said. But as of now, "soft CC", as most people call it, also stops creatures with any amount of HP. That's pretty much hard CC, in my opinion. It's something worth comparing with instakill.
Spoiler for Hidden:
A Bigger Bottle of Basilisk's Blood:
6
Spell
Target creature is disabled for 15 turns.
It's not instakill, but it might as well be.