well do you want too decide on who needs it=everyone. (i mean its a beneficial card anway) so thats not much use.
Take out those who dont need it=aether(immortal/immaterial) or gravity(high HP) or earth(high HP) or water(We already said) or entropy(can mutate) or time(rewind/reverse time) or light(heal)
so that leaves
really needs it due to low hp creatures
doesnt at all as it benefits from death and a creatures death doesnt really matter.
yes, cant find areason why not.
ditto
dittoAs you said life seems most sensible.
sorry before i forget you may want to take into account some of the numerous cards that start poison counters that are being made and how this would affect them i.e. tic toc, and ....others (there are more, Damnit! why cant i find them?)