The Meaning of Life, the Universe, and Everything.
Add a rare enchantment for weapons called lifeline. Basically, it has a chance to heal you every time you deal damage to any mob or animal or player. This is a pretty simple suggestion.Zombies with weapons that have lifelink will damage themselves when they damage you or a villager. Other mobs that are not undead can heal with Lifelink. Also, maybe you can add a curse enchantment that damages you whenever you damage someone else. It's like the opposite of lifelink.