1.) I don't really see why from a power perspective it would only work on enemies. Only 1 empire skill(imperial vengeance), a couple of tribunal ones (shackles/manacles/conscription), and one maran (eyes of flame) only function against a subset of people based on the cabals definition of an enemy. Your ability to swing really really hard isn't based on what you are swinging at, it comes from the swing itself.
2.) From a purely game balance design point of view villagers are highly feast or famine. They are ultra predictable because of their restrictions (no interesting progging/special gear, no preps at all, no magical transportation) being predictable lets folks engineer the high chance of landing a kill situations. The theme of the cabal powers is about jacking up offense and defense (as opposed to outlander which has much broadly utilitatian powers to address having fairly stringent limitations). When someone has a villagers number, there really is just not many options the villager has except to take it on the chin or avoid confrontation. Without the powers to boost their base offense (deathblow, trophy, critical hit, bloodthirst) and defense (field dressing, spellbane, resist, spell evasion, poultice, doubleblock) they would just be free meat for a pretty broad range. Now there are points on the scale, berserker gets 3 offense boosting powers, defender gets 3 defensive, and scout gets a utility and 1 each for offense and defense. That svirf axe spec berserker might be someone you can't beat solo as a muter, but it simply can't survive a bashing giant plus an armadillo (or healer, or bard).
3.) For the non mages being brutalized by deathblow if they don't have both parry/dodge as class skills then they have some sort of built in access to damage reduction. If you do have both of those then you likely end up eating less additional damage from deathblows than mages with just their class defenses up because of how many you parry/dodge vs how much it stings if it does land (far more frequently against mages with only one defense skill). A lot of mage players habitually prep for fights with villagers, non mages who invest as much time in leveraging preps also see a lot of return on that investment when fighting villagers.