Potential with Auchenai Soulpriest, but otherwise that's an ungodly bad drawback to put on a 5-mana 6/6. Zombie Chow's drawback is excusable because it's a 1-drop and you'll rarely ever heal your opponent for 5 with it. This will almost always heal your opponent for the full 8.
To those saying it's insane in Priest, explain how you're going to trigger the effect with Soulpriest. I just can't see it working with Lightbomb being out of Standard,
Well, pretty easy, when you think about it. You could deliberately attack with it after summoning the Soulpriest the same turn, and ensure that the Healbot dies. 5 damage turns to 8 damage pretty quick.
OR, as a possible Turn Seven play, you can summon the Soulpriest and target the Healbot with Shadow Word: Death. Bam. Instant 8 damage to the face.
Interresting in Wild Mode; Druid of the Claw will be much less of an problem for priest with him in our deck. This and his effect to deal up to 8 damage is really nice to have. But the statline could be better: a 5 mana 6/7 would not be too much for the negative deathrattle, but you can't have everything.
And even if the healing-effect of the deathrattle triggers: Holy Champion and Light of the Naaru really benefit from it. Healing the opponent is not really much of an issue for priest...
This could also lead to some insane burst-priest with Baron Rivendare; ok, maybe too greedy, but still: an Auchenai Soulpriest - Circle of Healing together with this card below 4 health, and a zombie chow wipes the board and deals 13 damage to the face... this is far from unreliable.