Could be a really good card, but you need some way to mitigate the downside (like Amethyst Spellstone), or some kind of reward for being at low health (like Molten Giant/Hooked Reaver). This would have been nuts in Evenlock.
Now that Warlock doesn’t have much healing, this is bad in Standard. Playable in Wild.
What wild deck does it fit in? Single target removal sucks in wild now unless it's 3 or 4 mana. There's way too much aggro in wild so running another board clear(such as Abyssal Enforcer) would be better still. May be good in standard if the meta isn't mostly aggro there too.
I think people underestimate how much healing warlock already has. I play a fair amount of plot twist warlock, and healing is rarely a problem: there's the Broodmothers (which can be shuffled in with plot twist and more copies added with Elekk), Applebaum and Deranged Doctor in combination with Dollmaster Dorian and Lord Betrug, and of course Zilliax, often magnetized onto a Mechanical Whelp/Dragon.
I really like this card. I think it's best compared to Siphon Soul: both cost 6 mana and destroy a minion, but IMO it's much better to leave a 7/5 body behind and take x damage than it is to heal 3. Obliterate on a stick, basically. I'm definitely going to try to fit it into as many warlock decks as possible!