| Gramps |
Extrapolating from Set's paradigm, this is what I come up with...
A good cleric, through divine grace, is able to tap more-or-less directly into the source of life, and "metabolize" it so that any living creature can directly benefit from it. However, when used to harm undead, this same life energy is essentially operating as a fire hose (much like Ironicdisaster details above). Too much of a good thing is, in fact, a bad thing.
Conversely, an evil cleric would essentially be drawing the ambient life energy into himself and making a "donation" or offering to the deity he serves (funneling that life force directly to his deity of choice in a sort of corrupt Thanksgiving... "Thank you, oh mighty Evilness, for the power you have granted your humble servant..."). When used to "heal" the undead, however, the way it works is as a perversion of what a good cleric does; the evil cleric steals and then "metabolizes" the life energy and immediately turns that same energy around and "feeds" it to his undead minions (or masters, whatever).
Looking at the abilities this way changes nothing mechanically, but completely alters the way we would view the game world.