Café de Nuit

Hey guys, I am planning to use the Deadly Presence feat (from Champions of Darkness page 41) with an NPC and I did some of the math and when I might be using it against the PC cleric, there is a good chance that subtracting the NPC's level and Cha Mod from the healing spell that there might be a negative number left over. Would you just count the healing attempt as nullified or would you deal the difference in damage based upon the feat description?

personally, I would just nullify it...just in the spirit it is supposed to be a healing spell

but then again...this is RL and twisting the spell to harm someone may be in flavor too