If B's an inconsequential stopover, then explain every other rodent in a similar ecological niche. I picked rabbits because they're cute, not because they live unusually unpleasant lives. Sexual selection can produce far worse than A (e.g. ram horns can grow through their skulls, gradually impaling their brains and eventually killing them). And the category of "distasteful" is very, very large indeed.

Nature is red in tooth and claw: trusting evolution to shape a better humanity in the absence of medical treatment is playing "look, ma, no hands!" with eugenics, retroactively justifying every tin-pot dictator's killing spree as a rightful bestowal of the Darwin Award. Medical care to prevent avoidable injury and death is good, actually.