Children are right little savages. My four-year old, propelling himself along on his brother's brick trolley, swinging his plastic cutlass over his head crying 'Catch the robber! Cut, cut, cut! Kill, kill, kill!' His eighteen-month old brother laughing, trying to run away but when caught seeming to revel in his pretend (and quite careful) chopping up.
Where do they get the idea that this sort of thing might be considered fun? We do discourage it, albeit half-heartedly now. But it seems totally innate and unstoppable. Thank God for civilisation.