So, currently, all healing spells in the Life tree states, "gains a bonus based on the time since the caster harmed any target". Wouldn't it makes sense for that limitation to not apply for undead? Undead are the very opposite of life, so it seems that perhaps there should not be a penalty for destroying them - and perhaps even a bonus. This seems particularly true in the case of Banish Undead. The life tree itself has a spell that damages only undead, yet that counts towards getting a healing penalty.