if it's OP maybe it's better something like "gain health equal to creature's health before dying"
this way if you kill with venom you gain only little hp
if your opponent can kill an armagio with gravity pull in a turn, well.. i guess 20 hp won't be something wrong to gain
if your opponent leave armagio with (for example) 5 hp and then kill it next turn.. well.. you gain 5 hp
i think this way is not so OP