I would think a simple limit on the amount of piety though life steal you can get per monster would help curve bonus. Something like 1 hit per monster for 1-2 piety a hit.
Additionally I would also limit life steal to never steal more than the amount of damage you do, while still allowing overkill damage to count, making it a valid strategy to leave low level monsters to help heal you a bit during the boss fight, much in the same way paladins recover 1/2 their health when killing undead.
Or, change life steal to grant 1 per character level.