I would like to think that an iconically good creature such as a unicorn would not easily follow this path, even in the case of the violent death of their mate. A subset may fall into darkness, but I would not think all.
Not a bad critter, but I have a problem with natural creatures which specialize in human prey.
A human population providing enough food for a bunch of squirrels to evolve eating them is not very likely - the group would either move away or attempt to exterminate the squirrels. Either would not provide a very stable diet. A magical creation or unique mutation I could believe. Go to Comment